At today's prices you should expect to get close to $500(US) if your Pro1 is in as good of condition as you say. Top dollar is usually paid when purchasing from an original owner such as yourself. Check out:
http://www.midiwall.com/usedgear/ I, being the owner of a P10, will have to disagree with 800dv's remark about SCI's reliability. This P10 has only been in the shop once since I purshased it, also from the original owner, many years ago and then it was only for inspection and calibration. However, the *original program memory battery died -last year- and had to be replaced. While time may have barred the P10 from being gigged, it is still quite healthy in a studio environment.