There are readily made FPGA clusters of course, but they usually include a lot of features that you don't need (like high speed interconnects between the individual FPGAs), and you often won't even find prices for them, as this kind of stuff is usually in the >$100k range.

Building your own one (if your have the experience) might be as "cheap" as $10k per 6GH/s or something in a 3U case.