I fear this will be a bit more expensive then GPU (per Hashrate).
Mainly because of the expensive high speed mem interface (max. DDR3 with FPGA currently afaik).
And the FPGA with a lot of on chip mem are very expensive. Have done some small research too, doen't
look to well currently...

There have been some discussions and calculations about this already - either in the altcoins subforums
or at litecointalk, can't seem to find it currently...