Congrats!! Treadles are such fun to use!
As for pricing, it's all over the board and would depend on the machine that you get, it's condition, etc. I've paid as little as $50 for my Davis that needed work, and as much as $400 for my newest. (Keep in mind that I strongly resist even looking at anything marked over $100, but that last one was calling my name)
I recently read that there is no treadle in a "standard" treadle cabinet that is worth more than $150. (Antique experts) I would think about the same for parlor cabinets, and the drawing room cabinets are worth a little more. If the machine has Mother of Pearl inlay, good decals, you'd expect to pay more, scratched up sphinx or redeye decals less....
It's really hard to price a machine, but keep in mind that "sentimental has no value".