Gsp
I'd really look closely at a .22 GSP made in the early to mid seventies. There was a rash of failures of the bolt housing due to a stress-raiser in the machining. (The bolt housing is the steel piece in which the slide reciprocates. It is the part with the serial number.) The failure manifested itself as a crack at the bottom right front of the cocking-ear slot. If the pistol continued to be used, cracks developed on the left side in the same relative spot. The failures did not develop until after several thousands of rounds had been fired. Attempts to repair the crack by welding proved largely unsuccessful. Replacement housings had modified machining that eliminated the stress-raiser. Walther does not carry replacement housings. Earl's Repair in the USA might still have some.
Other than that one model, the GSP has an excellent reputation among bullseye target shooters (both NRA and ISSF). They are very reliable and handle a wide range of standard velocity .22 ammo.
If the gun was not of the problem era, anything up to a $1000 seems to be the going rate. A problem era gun needs to be scrutinized closely, even if it appears in 98% condition. You would be buying a pig-in-a-poke.
As mentioned, the OSP (the .22 Short model) has lost considerable value because it doesn't qualifiy for ISSF competition under the current rules [which brings up the question as to whether it should now be classed as a prohibited, 12(6), because it is no longer used in international competition]. Personally, I don't expect an OSP is worth more than $500 these days - if that much - as it is now of interest only for casual shooting (plinking). (Well, with a change of trigger and grip, it could be used for the NRA Conventional Pistol Gallery Course - but that trigger and grip could set you back an additional $250 to $550.)
M.T.