Local shop is always a good first stop. Put your eyes on things, check out the fit, talk to the pros... in my experience when they hear it's for cave use, they tend to not push the super pricey stuff (since it will get torn up over time), but they do recommend more stretchy stuff for greater mobility (XSPAN, Super-Stretch, Hyperflex- depends on the brand).
I've also had really good luck calling scuba . com. I get a thickness recommendation from other cavers, and get the fit info from the guy on the phone at scuba. Height, weight, chest measurement, and the two suits I've got from them both fit just fine.
As for price, you can get a full body 7mm for $90 up to $500. No way I'm going to spend $500 on something that's going to get scuffed up in a cave, so I went for the middle ground-
http://www.scuba.com/scuba-gear-101/050 ... tsuit.html$225 and I've been very happy with it. Fairly stretchy and comfortable for a 7mm. I wear soft knee and elbow pads over the suit, and it's still in pretty good shape so far. I've seen some cavers wear a pair or two of shorts over the wetsuit to provide a little butt armor. Warm and toasty when you're up to your neck in 47 degree water.
My 3mm is for a cave where it's dry, dry, dry, go through some wet spots, dry, dry, crawl though a shallow steam, then more dry stuff. There I wear my wetsuit, knee and elbow pads, then my cave suit over that.
Like Chad said, there are tons of options and you don't want to over or underheat, so talk to people who know the cave(s) before reaching for your $$$.