Usually each generation has a low end, medium, and high end chip. The high end chips tend to be loaded with lots of gee-wiz bells and whistles which take up lots of silicon real estate but don't do crap for mining. This is why Spartan-6 despite being a low end chip is economical for mining.
Artix-7 is likely going to be the "next gen" chip for mining. http://www.xilinx.com/products/silicon-devices/fpga/artix-7/index.htm
It is low end part like Spartan-6. Eventually (supply, demand, yields, all that good stuff) it should be available at sub Spartan-6 prices. So we are talking <$150 maybe <$120 per chip.
Its largest model has 350K LUTs vs 150K on Spartan which should make 3 hashes per clock possible (or at least 2). If routing w/ 4ns critical path is possible that means 250 Mhz * 3 = 750 MH/s. Granted that is very optimized so first versions won't be that good. It will take some fine tunning of bitstreams.
To put that into perspective if
* Artix-7 350K LUT is available for <$150
* It can be clocked @ 250 MHz with fully unrolled hashing bitstream.
* You can fit 3 fully unrolled SHA-256 double loops in 350K LUTs
It would be possible to build a dual chip board getting ~1.5GH/s @ ~40W for ~$500 .... someday.
Note that is just back of napkin estimate based on the the relative performance of the chips. Until someone gets one and starts trying routing no way to know for sure but it should be > 2 MH / $ and > 30 MH / W.