Galil is a great supplier of these.
http://www.galilmc.com They have the pricing on their site. If you get one from eBay, do your homework. Some of the controllers on eBay are OEM spec controllers that Galil made for many different machine companies. These can get quite custom and you may not get what you think you are getting. And Galil will NOT support them. For instance, I have seen a 5 axis controller with ABCD and F (skipping the E axis). And I have also seen controllers with custom firmware that is so specialized that it will not work with the regular Galil command set. So be careful.
The good deal days are gone on eBay. Most of the time, you can get a brand new 41x3 controller for less money than people want for the older Optima controllers on eBay. Becuase the Optima controllers used a very special and expensive interface cable that you just have to have and then you will also need an ICM2900 (Breakout board) to go along with it. The new 41x3 econo controllers make use of cheap DB style connectors making it relatively easy to roll your own cables.
Every now and then there might be a good deal on eBay. But it's always a risk.
Don't just look at controller prices. Make sure you account for everything that is needed.
Steve