As has already been stated, it's difficult to estimate a value as there isn't anything to compare it against. Many collectors, including myself prefer short-run production items rather than prototypes when looking for rare pistols.
So, the pistol will appeal more ro a niche market and would probably do better if it were sold at auction where it will fetch whatever someone is willing to pay for it.

Good luck with the sale if you decide to part with it.

Brian