There are a few cams that are harder to find and/or more desirable than the others. The ducky cam for the later 700 series Touch&Sews is one that comes to mind.
I've bought all my cams at thrift stores but that's very hit & miss and not a reliable way to price them. We all love a good deal and I'm a bottomfeeding cheap S.O.B. by nature but I think it's important to remember that there's a difference between the exceptionally low prices we like to brag about here and what the full retail prices really are.
It's been a little while since I looked. The last time I looked it was possible to buy a basic set of cams for under $20 before shipping. It can cost more if it's a 400 or 500 series set in the right box.
Rodney