It can be done, my 2500k only costed me about 210$, the GTX 560 Ti about 150$, the mobo (Asus Maximus IV Gene-Z) about 195$ and everything was new. You might be able to get a 500-600 Watt PSU for about 45$ (maybe used, but used stuff generally also works great).
This build can handle every game you throw at it smoothly, everything at max settings ofcourse (including anti-aliasing).
And all that when they just released, the prices probably will have dropped by now though (I got the parts from auctions on eBay, that's how I got the pretty decent prices).
EDIT:
http://www.newegg.com/Product/Product.aspx?Item=N82E16819115072 2500k, easily overclockable to 4.5GHZ 220$
http://www.newegg.com/Product/Product.aspx?Item=N82E16813131806R Asus MIV Gene-Z/GEN3; it does the job, it does it really well 128$
http://www.newegg.com/Product/Product.aspx?Item=N82E16814500196 Zotac GTX 560 Ti (it does the job, but for ~50$ more you could get a 570) 210$
http://www.newegg.com/Product/Product.aspx?Item=N82E16817182199 Rosewill PSU, it's not bad but it'll get the job done. You probably will be able to overclock too (atleast I could with a 560 Watt one, my systems usage never is above 400 Watts) 55$
http://www.newegg.com/Product/Product.aspx?Item=N82E16835181012 This is the heatsink I use, it keeps the CPU as cool as it should be 33$
It's a bit more than you'd want to, but just check on eBay and you'll probably find something decent around these lines.