Pax, I offer you might be missing a few points, and although it is tough to find a price, I feel you are way off in saying a C-17 is less than 50% of the A-400 purchace price. Way off.
A-400 customers mostly needed a C-130 and C-160 replacement/augment, with a bit more capability than what those have, not the huge leap the C-17 brings. For many scenarios you do not need the capability the C-17 brings. The A-400 is a "tweener" between the C-130 and the C-17, and for some customers that makes sence, especially for many of the European customers that are not flying continental distances that often. The UK has decided they need both (and that makes sense for them).
The A-400 claims lower operational costs. I would not want to pay the refueling bill on either, but a C-17 burns more fuel.
The 400 is likely better for tactical airstrips. Although the C-17 touts rough strip capabilty, it is mostly used as a hub and spoke type transport. What you read and whom you believe may influence you thoughts here.
Political and industrial implications are also important. "Buy American" is not always the answer- and I am one (American), but understand that.
You don't need a semi-truck (lorry for my UK friends) for every run to the home improvement store....