There are definitely cheaper dry suits out there. Seems there are some deals out there on Bare Nex-Gen suits if you're looking... Check out this link where the base price on one of these is $640.
ScubaToys - Dry Suits
I've never worn one of these, but many reviews I've read have stated this is a quality product. Search the forums for more info. I will say this, though, I tried on a White's Fusion dry suit the other day and that was absolutely the most comfortable and easy to don dry suit I've ever seen. It's a bit more money, but holy crap if money were no object I'd be diving one of those every time.
As far as undergarments, you could go cheap-o and just wear sweats. Many folks will tell you this is a no-no and they're generally right, but it does work. You won't be as comfortable and they won't wick moisture away from your skin like modern undergarment fabrics do, but it can be done, and it's still warmer than diving wet. Old school tight-wads like me did it for a long long time, and I'm sure there's others out there who still do.
One other thing I'll say though, the investment you make in a dry suit is a good one. If you take care of it, it will last you a long long time, ideally much longer than a wetsuit, and you'll be much warmer. If you do multiple dives in a day you'll have LOTS more energy at the end of the day, which was the biggest difference for me. For me, before the dry suit, boat trips usually meant 3 dives in in water that was in the mid-50's and by dive #3 I was shivering before my fins hit the water. I was usually just surviving and toughing it out rather than really enjoying it. It was a draining day. Once I got the dry suit, it was a much different story. Warm all day, and plenty of energy to go out that evening. There's something to be said for this and a value that might make the extra money worth it.
Hope this helps!