Being as they're out of warranty, and previously owned (used), my guess is you'd need to start at some price less than the current price for new ones.
I got my new RSV2 for $80 18 months ago, but that was a terrific price available for a very limited time. It usually runs two to four times as much.
Linens-N-Things is currently advertising RoboReptile for $35
ScottE -- collecting and building robots for 40 years - details in profile