The 8150 is more future proof? Not sure on that. The 2500K runs LGA1155 which will support Ivy Bridge, coming out next year.
As for the Windows 8 comment, as far as I checked any CPU is compatible with Windows 8, including ARM based CPUs. And from what we have seen with the developer preview so far, 8 does not increase performance enough to make it better than the 2500K performance wise, especially in games.
As for the OP, yea there are cheap AMD mobos out there. But you can get a P8Z68 for $150-$200 depending on variation and most AMD mobos of that quality also cost $150-200 depending on variation. You can always go with a cheaper Intel mobo but why go cheap on a very integral part of the system? Get a decent $150ish mobo for any build, that way you have a decent part with a ton of new features and better compatibility.
If I had the money to buy a system right now it would be the 2500K and a P8Z68-V Pro or Deluxe, depending on how I felt. And thats based on the performance of the chip for the buck. The 2500K is just amazing performance wise and overclocks very well on air cooling. Plus its around $200ish normally.
Also, don't cheap out on the PSU either. Get a Corsair, Sea Sonic or the like. The cheaper the PSU, the better your chance of frying other parts.