Generally, historically England can be said to have been sparsely populated; this put an upward pressure on wages, meaning that skilled laborers or educated clerics could command a premium for their services. However, I also suspect that England's historically numerous Abbeys might also have allowed for a good standard of living amongst the high-level clergy, and ecclesiastical posts in England might have also been paid better because their distance from Rome meant that they needed to provide some additional incentive for people to want to go there.

Pro-capita wages aside, in absolute terms England was a smaller economy than others on continental Europe prior to the advent of Atlantic trade (raising the suspicion that if true, William's chaplain's words might be fueled by some ulterior motive). The largest data set on economic development from the fall of the Roman Empire to the modern day was published in 1990 by the British Economist Angus Maddison (updated by Bolt and Van Zanden in 2013). The data for early years is sparse, and only when enough evidence has been transmuted down to us for a single year have estimates been made, but in any case here's the chart for a number of countries you might find interesting (hopefully it translates correctly, I um, don't know how to make charts on Reddit, in any case the full list is free to download here)

Year - Germany - Italy - Netherlands - England

1000 - (null) - (null) - (null) - 757

1150 - (null) - (null) - (null) - (null)

1280 - (null) - 1,620 - (null) - 677

1300 - (null) - 1,515 - (null) - 742

1348 - (null) - 1,515 - 876 - 786

1400 - (null) - 1,751 - 1,195 - 1,099

1450 - (null) - (null) - 1,373 - 1,076

1500 - 1,146 - 1,533 - 1,454 - 1,086

To build on what /u/AlviseFalier has already said, early medieval England benefits from three important advantages: Landscape, resources and location.

During much of the early medieval period, England's landscape provides several economic benefits. One of the most important factors when compared to the rest of Europe is that England is an island. This may seem obvious but shouldn't be overlooked. By the eighth century, the historian Bede is already postulating the idea of a gens Anglorum, and due to its isolated nature, England is able to form a series of relatively stable, centralised governments far earlier than in the rest of Europe, before finally unifying in 925. This relatively stable political situation helps bolster trade and the growth of settlements. It also provides a great deal of security. Though England has to deal with the Viking threat from 793 onwards, and the economy suffers as a result, by the 900s, the English kingdoms are centralised, militarised and on the offensive, both militarily and culturally. The network of burhs - defensive linchpins which provide garrisons at important road junctions, river crossings, etc. - also provide the foundations for a network of secure sites on major routes which allow trade and settlement to flourish. Chester, for example, had been a desolate and abandoned legionary fortress until its defences were restored by Æthelflæd of Mercia in 907. Within 5 years, it was the most productive minting site in England, and the safety provided by its garrison and those of nearby burhs allowed for the rapid development of high-value farm settlements across the Cheshire plain which can be witnessed in the archaeology. For much of the tenth century in particular, England is relatively peaceful. The expansion of Wessex and Mercia into the Danelaw bring peace to their heartlands, and following unification, a national peace is eventually brought about under the reign of Edgar, who is also famed by the Anglo-Saxon Chronicle for preventing any hostile landings in England during his reign. In contrast, mainland Europe is plagued in this period not only by some Viking activity, but also by Magyar raiders which ravage across Germany and, according to Regino of Prüm, even manage to penetrate as far as France, carrying off vast quantities of loot, and burning what they can't take. Following the Conquest of 1066, this is a trend that continues, especially once the upheaval of the conquest campaigns has abated by the 1070s-80s.

The south east and East Anglia in particular are large areas of flat, extremely fertile soil which are well suited for growing crops. In the South West, Wales and the Midlands, there's lots of gentle hills with high rainfall, which makes for perfect cattle and sheep country. England in the medieval period is famed for its wool industry, which it exports en masse to Europe. Dorset is a fine example of a county with many 'wool churches' - churches far larger and more ornate than they need to be to support their local community, but paid for by the profit from the wool trade.

England is also criss-crossed by a particularly high number of rivers, which provide an easily accessible transport network between towns. Particularly dense areas of settlement occur, for example, along the Thames, the Severn and the Trent Valley, where the waterways facilitate the easy movement of people and goods.

Britain is also surprisingly resource-rich for a small archiapelago. In the early medieval period, this is most prevalent in the amount of silver available, both from English-controlled Welsh mines, and from the 1100s, from the new city of Carlisle. The prevalence of silver means that Anglo-Saxon England is famed for its coinage, which from the 840s onwards, retains its weight and silver content relatively stable for over 200 years. This stands in marked contrast to European coinages, where political upheaval and the privatisation of minting in Carolingian France meant that the value of the coinage fluctuates wildly. The coinage is kept under particularly tight controls by the crown, is tightly regulated and undergoes regular renewal and reform. This is aided by the fact that England is in island - by the late ninth century, Alfred the Great's government has tight controls in place which seize Carolingian coinage at the ports, often reforging it into higher-value English coinage, or simply restamping it if its silver weight is sufficient. English coinage is widespread in Ireland and Scandinavia, with distribution patterns illustrating that was in use there as the de facto everyday currency amongst the Hiberno-Norse and further afield, rather than as bounty from raiding. The archaeological record also shows a great flourishing of material culture in this period.

Following the conquest, in 1092, William II establishes a royal lodge at Carlisle, leading to the discovery of vast quantities of silver. Mining begins, and soon Carlisle is a spectacular boomtown which draws attention from across Europe for the sheer volume of productivity. Having been desolate wasteland at the end of the eleventh century, by the 1170s, Carlisle's castle is deemed 'magnificent' by Jordan de Fantosme, and forms the linchpin of the defences of Northumbria against Scottish invasions of England, fighting off two sieges by the Scottish army during the war of 1173-4.

England's location is also vitally important to its prosperity. Key to this is its coastline. In the seventh and eighth centuries, the south and east coasts are part host the rapidly developing network of wic sites which provide a flourishing trade network between England, the Netherlands and the Rhineland. These are sites such as Southampton (Hamwic), London (Lundenwic), Ipswich and Norwich. In the tenth century, the cities of Chester and Bristol develop rapidly into major trading centres as the centres of trade between England, Wales, Ireland and the Scottish Isles, and even as far as Scandinavia.

Some sources:

Keynes and Blackburn (1999), Kings, Currency and Alliances

Britnell and Hatcher (1996), Progress and Problems in Medieval England

Gillingham (2000), The English in the Twelfth Century

Higham and Ryan (2013), The Anglo-Saxon World