Both Altera and Xilinx sell FPGAs which are in some sense designed for DSP applications. How large an FPGA you need depends on other parameters which you have not specified, such as how many bits per sample, and how many clocks are available per sample. Altera and Xilinx offer development tools which have a variety of automated tools to help you do FPGA-based DSP, of course, none are as good as hand-coding. For many applications, though, they are enough.
Regarding cost, generally the older chips (2 or more generations old) will cost more per CLB than new chips. Then again, there are more development boards and tools available to support the older chips.