The US is richer, and has a larger GDP as of now (post Dollar run and Euro drop) than the European Union. The US has about ~40% of all private wealth (globally private wealth is around $270 trillion, with US households holding $86 trillion [1] of that; then furthering that are US corporate assets, which are far larger than European corporate assets).
By comparison private wealth in all of Europe is about $60 to $65 trillion. [2]
Better yet, why does it have to be an "individual nation" thing? Why can't all (or even just some) of the developed nations collaborate and pool funding and have a bigger team to work on the problem?
Sure. And we definitely need some kind of motivation to focus our efforts on space exploration (it's very noticeably become an afterthought ever since we stopped trying to compete).
But if we could somehow find that motivation, it would be far more productive to work together -- we'd have a lot more manpower and a higher budget that way.
By comparison private wealth in all of Europe is about $60 to $65 trillion. [2]
[1] http://www.federalreserve.gov/releases/z1/Current/z1.pdf
[2] http://www.reuters.com/article/2014/10/01/us-europe-wealth-s...