The CDX is a pretty solid unit with very good audio, but if you're looking for a lower priced entry point you might be better off with just a Genesis and Model 2 Sega CD. I normally see a CDX going around $100 or more, there's someone local to me who has one for sale at $125 but I've already got a working one.
I would say your best bet is to try and get one with a lot of games. I picked up mine for about $100 with 15 games several years ago, but prices have risen since then.