Razib discussed this recently (partially at my prompting). Consider this material from Azar Gat’s “War in Human Civilization” supplementary.
“[…] nor was investment in colonial markets sought by investors in preference to diminishing returns on investment in the developed economies. The more developed the country of investment the higher were the returns, with the new African acquisitions bringing the lowest returns. […] This is demonstrated by the fact that the fastest growing new economic giants of the late nineteenth century were the USA and Germany, which, despite their new colonial ambitions and minor acquisitions, were the least of the colonial empires […] the largest and fastest growing colonial empires, Britain and France, suffered the greatest relative decline in economic status among the great powers during the era of the new imperialism. Indeed, with the close correlation of economic and military power, the empire’s poor military contribution mirrored the economic one. Metropolitan Britain incurred 80 percent of the casualties and 88 percent of the costs of the First Wrold War, with the remainder, the imperial share, taken mostly by the self-governing dominions [Canada, Australia, and New Zealand]”
Gat explains the “scramble for Africa” by virtue of the strategic threats (rather than economic incentives) Britain felt as a result of Russian ambitions in the eastern Meditteranean and central asia (and later a threatened alliance between Transvaal Boers and new German possessions in south/east Africa). Britain’s seizure of Egypt to secure the Suez frightened France into seizing territory, setting of a chain-reaction of land-grabs.