So if you were to get a cheap license via a service like this, do you get access to the VHDL or equivalent or could you extract it in some way?

Probably not. Depends on how the core is distributed. Either you'll get HDL or netlists, and they may or may not be encrypted. Obviously the synthesis software has to decrypt it to use it, so like all defective by design DRM it doesn't make it impossible to get at the code, it just makes it more difficult. However, a netlist is just a schematic, so you would have to 'decompile' that back to HDL (and lose the names, comments, etc) if you want to modify it. It's also possible that you would only get the binary FPGA configuration file (this marketplace seems like one more for complete appliances and not IP cores) so you would have to back a netlist out of that somehow and then reverse-engineer it from there.

there are "encrypted" hdls, but encrypted no more than dvd discs. Find a right irc channel and ask for keys