The last set I sold on EBay went for $212 for a working 3 board set (MPU, ROM & I/O boards). That was probably exceptional for the current market. There is a set on Ebay right now for $175 that has been there for a while, but the guy claims they were "fixed" by Eldorado (no comment, but do a search for Eldorado in this forum and the repair forum), but he can't test them now.
If it is a proveably working set (i.e. you have pictures of it working), you should be able to get at least $150-175 on Ebay, I would guess.
ken