As most of my usual sources are temporarily unreachable, I'll address the aspects of imperial wealth that I do have sources for.

The City of London and Empire as Profit by Position

Davis and Hutchback cite as one example of Empire's relative unimportance the fact that colonial finance did not dominate the capital markets in London. The problem with this is that London's position as the centre of the world's capital markets owed itself partially to empire. The vast reams of information on land, people, and finances produced by imperial pursuits provided prospective overseas investors with more information than they could hope to find in other capital markets [1], which in turn only increased the expertise and capital located in London, and thus made the City the thoroughfare of investment into the entire extra-European world instead of just Britain's own possessions.

This situation extends itself to many other aspects of Britain's wealth in the Nineteenth and Twentieth centuries. The African colonies, for example, may not have contributed much to the balance sheet of the Treasury in hard tax terms. However, their strategic position in relation to the security of the Suez Canal and Cape of Good Hope offered a benefit for the Royal Navy, and thus the British economy as the two major sea routes from Europe to the East were more firmly in British hands, and easier access to their ports offered the British Merchant Marine a competitive advantage that helped British business as a whole.

Then there is the matter of the White Dominions, whose legal and cultural relation to Britain is complicated to say the least, and their "burden on British taxpayers" in relation to the trade relationships created. All this to say nothing of the massive Atlantic Economy that the British Empire indirectly created through the United States, both in its origins as a British colony, and the millions in Sterling invested in the country over the course of the Nineteenth Century.

The Empire's main contribution could thus be argued to be its role in Britain's status as the number one world power and centre of the world economy, but the contribution of that status to Britain's domestic and foreign markets is very hard to quantify. And since it's hard to quantify how much of those markets owe themselves to empire, it's hard to quantify how much or little of the profits of empire really contributed to the wealth of the average Briton. It is much the same problem as trying to quantify the cost-benefit of the military and treaty commitments of the United States in the post-World War II world.

Tax Burden

Like the whole project of Empire, its tax burden depends on which part of it is being discussed. The White Dominions for example, de facto had their own governments and armies by the First World War. But I want to take this moment to mention the Jewel in the Crown.

The British Raj, strictly speaking, was not paid for by the British taxpayer. The Raj funded itself through India, and this allowed Whitehall to increase the size of its military commitments without levying extra taxes at home, or even putting the expenses down to the Treasury. Instead, they would order the Raj to raise regiments, and naturally, as part of the Indian Army, the Raj would pay.

In short, as far as Davis and Huttenback go, I wouldn't dismiss them at all, but I also wouldn't take it as gospel that the average Brit didn't benefit from the Empire. Perhaps they did not in direct profits from the colonies themselves, but perhaps they did in the status and competitive advantages the Empire afforded British enterprise.

Hopefully this helped, even if it is far from my finest work, and it wouldn't surprise me if someone with a more complete understanding of Mammon and the Pursuit of Empire comes to tear me apart on the whole thing.

[1] - You can see some of the extent of these archival treasure troves simply by going to the National Archives and searching the name of a colony. The amount of documents that will pop up is staggering. For example, the search term "Sierra Leone" brings up nearly 18,000 results, including journals and correspondence by governors and maps produced by the Colonial Office for all kinds of purposes.

Sources

Darwin, John, The Empire Project: The Rise and Fall of the British World-System 1830-1970, (Cambridge, 2010)

Cain, P. J. and Hopkins, A. J., British Imperialism: 1688-2000, (London, 2002)

Are you asking about the Empire (16th century to later 20th) or just about the relations of wars and the Empire. I am a bit confused to exactly what you are asking.