The answer to this question hinges quite heavily on where in c.1000 Europe we are. By the end of the 10th century, and in fact since about the 8th, the economy of England had been surprisingly significantly monetised. Although elements of a barter economy were still widespread, the use of silver penny coinage was commonplace. Some prices were legally protected and the coinage had remained remarkably consistent in content and value for approximately 200 years, in marked contrast to Carolingian coinage which fluctuates massively.

Although silver coinage was widespread, the gold coinage of your question was another matter. A single gold mancus of Coenwulf of Mercia (c.825) exists in the British library, and the will of King Eadred tells us that he wanted 10,000 made, but there's no evidence yet of these existing. While it could be that simply none have been found, it also suggests that these were minted to be presented as a ceremonial gift and not intended for general circulation. Finding one, therefore, could be a significant windfall.

It's tricky to value a coin not necessarily intended for circulation, so in this situation, since the penny was referred to as the Roman denarius, we'll model the mancus economically as equivalent to the later pound, or libra, in that it was worth 240 pence. While 240 pence today won't stretch particularly far, 240d in the 900s is another matter altogether. Æthelstan's Grately (II) legal code of c.826 sets a number of economic regulations. All trade of value over 20d must take place in a burh settlement; cattle are valued at approximately 10d a head. If you took your gold coin to the nearest burh (approximately 20 miles away), therefore, you could use it to become quite tangibly wealthy.

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

Naismith (2015), Early Medieval Monetary History: Studies in Memory of Mark Blackburn

Molyneaux (2016), The Formation of the English Kingdom in the Tenth Century

Ireland around this time was essentially a non-monetary economy - coinage started to be produced in the Scandinavian colony of Dublin only three years prior (in 997) under the reign of King Sitric Silkbeard, but this currency would have probably only been in circulation among the Norse population and have been used in trade with contacts across Western and North Europe's coasts.

In the Irish interior, however, coinage was not produced or really used as arbitrary units of value which was guaranteed by a central state authority because there weren't really any states. Instead, the early Irish economy was a sort of regulated bartering system whose primary unit of value was cattle. To a modern reader this might sound goofy, but the supply of cattle was pretty much fixed which meant that its value couldn't fluctuate dramatically through inflation (you can't mint new cows whenever you need them). Precious metals, presumably including coinage, had their worth determined by their weight rather than a fixed value and that worth would be set against types of cattle, slaves, jewelry etc.

I'll have to deviate a bit here, because Irish legal tracts don't actually seem to talk about gold as a currency, and archaeological evidence shows that silver and bronze seem to have filled the role of "precious luxury metal" that gold served elsewhere. So for the sake of this little writeup, let's imagine that some impoverished Irish freeman is ploughing his field one day and comes across a large, silver coin of Roman origin. There are numerous archaeological deposits of Roman silverware found across Northern and Eastern Ireland, probably sent as "gifts" to Irish chieftains by Roman administrators in Britain to pacify them, while some others appear to have been the property of Irish people who had served as auxiliaries in the Imperial Roman army and retired back to Ireland will all the goods they had accrued during their service. That being the case, let's imagine that our medieval Irish farmer has found not just one coin, but a whole hoard of them that weighs about one ounce.

In the Irish economy, an ounce of silver was what was known as an ungae and was the equivalent to a single milk cow. Besides that, he could buy eight yearling bullocks, four yearling heifers, three two year old heifers or two three year old heifers. Increasing your own herd had the advantage of giving you the ability to rent out your livestock to other farmers, creating a patron-client relationship between the two parties. In exchange for cattle (particularly milk cows), the client was to supply their patron with processed food products like cheeses, bread etc. as well as manual labour used to improve their patron's farmland. They would also perform military services for the patron, guarding their local territory against cattle-raids, which would have been a real concern in a stateless society where your wealth could be taken away in a single raid (or alternatively, he might organize his own cattle-raids into neighbouring territories!).

With a substantial herd and numerous clients, a common freeman could increase his rank and become known as a boaire (lit. "cattle-lord") who had a higher legal standing than most commoners. Gaining the status of boaire was also advantageous for one's lineage, as that man's grandson would be initiated into the lower echelons of the aristocracy later on. So if our farmer is forward thinking, he might invest in cattle and increase his herd to establish himself as a wealthy freeman with many clients in the hopes of establishing his own aristocratic dynasty.

However our lucky farmer's options aren't just limited to livestock; literary and historical sources shed some light on the cost of certain goods. He might also have been able to buy a riding horse, four healings by a physician, one or more poems about himself composed by a professional poet (fees for poems is mentioned in saga literature buy the exact cost isn't specified) or an ostentatious brooch manufactured by a skilled craftsman. These are only services mentioned in legal and literary texts, but we can imagine that an ounce of silver could get you any number of services provided by professional craftsmen so long as the object wasn't out of your remit as a common farmer (swords and other martial instruments, for example, are lordly items that seem to be limited to the aristocracy).