FPGA dev boards aren't that cheap, particularly one that can synthesize a 64 bit multi-core RISC-V CPU (in fact, there is one that can do it and it costs nearly $3000). The other problem with synthesized CPUs is that the clock speeds are very slow, unless you pay a great deal of money for the FPGA.

They're a lot of fun though - I have PicoRV32 in a very low end Lattice FPGA which cost, like $30 I think? It can only run single C programs, not even an embedded kernel, but I love playing around with it.