At the 51 core in the code library which is pipelined and runs at 1 clock per instruction(most of the time?)
You wont get much change out of $5000 for a fast core from a commercial vendor.As for which fpga to use ive had that core running at 80 Mhz in a xilinx spartan which costs £25.
As far as verifiction is concerned ive written a testbench which tests every instruction in modelsim and gives a pass/fail result.
Ive also written a perl script which looks at your assembly code and adds/removes hardware from the core as and when its needed so if for example you have a program in mind that you want to run on a fast processor and it doesnt use any divisions then the hardware division unit isnt compiled into the fpga saving space in the fpga and improving clock rate.