Cheaper FPGA for fixed designs?
There isn't really any technology that competes with FPGAs by trading off field-programmability vs cost. There are ways to lower the cost of your FPGA, though:


Use the slowest speed-grade part your design can work with.
Use the smallest gate-count part you can comfortably fit your design into (but it is wise in most cases to retain some unused resources to allow for future upgrades, and to avoid absurdly long synthesis times)
Buy as many at a time as you can of a given FPGA part number. IC prices can go way down as volume goes up. If you have more than one project going, consider using the same FPGA in both to increase the volume you're purchasing of that part.
If your purchasing volume is more than maybe 20 pieces at a time or 100 FPGA's per year, contact the FPGA manufacturer directly and negotiate the price. Don't just accept the list price from a distributor.