There are a range of Web Sites that give away FREE IP cores but they're unsupported. This means that there's no guarantee that they function correctly and nobody to complain to. So BEWARE! 

Altera (www.altera.com) have their own RISC Based Microprocessor IP Core called NIOS that integrates into their own family of FPGAs. These are FREE but can only function within Altera devices. They simply recover the cost of the IP Core development by selling lots of their devices.

IP Cores that are purchased usually come written in VHDL for portability which enables them to fit into any device large enough to hold the design.

Designing your own is perfectly feasible. However you must consider not only the time taken to develop the design but also the time taken to fully evaluate it.

Simulation is the normal method used for evaluating logic circuits. However with a microprocessor design, inputting stimulii that resembles an executing program would be immense! Therefore the evaluation would have to be done for real with the IP Core implemented in hardware and executing a real program.

However, where would you look when the program of an IP Core microprocessor CRASHES? Debugging would be extremely difficult!

This is the reason for the high purchase price - you're getting a circuit design that WORKS!