"I like FPGAs, but I doubt they will ever become widely deployed. They pay ~20x overhead, so any algorithm that is a good fit for them becomes a new instruction in the next CPU generation. The reprogrammability is only a feature in highly constrained (i.e. niche) environments."
http://www.reddit.com/r/IAmA/comments/1yj77b/as_requested_i_...
eg for something very simple you can do it for thousands or tens of thousands...but in that price range it's probably still more economical to implement in software or do it discretely with SMT unless you are very sure about the existence of a market.
However, algorithms are so rarely the stumbling block for applications; data storage, management and communications are. The major exception is graphics, hence the GPU.
The money is a huge barrier to entry for hobbyist types, but if we're talking about commercial stuff, it'll probably cost you a lot more to pay for the engineers who are competent enough to implement something that will actually work than it will to do the fabrication.