German police surplus PSP's. Most of the (unrefinished ones) sold for around $900 from the dealers. Now that no more are coming in, people are successfully scalping them for up to $1,250. Fair value is about $1,000, but people do pay more.
Non-surplus PSP's. There are just so few of these. I bought this 1980 dated one a few years ago, unfired, with original cardboard box, two mags, and German manual for $1,500. I thought that was a hell of a deal which would never come around again, and so far I have been correct on that!
P7M8. I bought this 1989-made one for $1450 on the EE with no box. I can't remember if it was one mag or two. I felt that was a fair price, but I see people all the time trying to get like $1,900 for their P7M8 with a fair bit of usage. Fair value on M8's is like $1,250 to $1,650 depending on wear and overall desirability.
I later bought a minty one with one mag from Electrobug with no U.S. markings for $1,850 I think. Electrobug had another one almost the same but with two mags, which I think he eventually sold for around $1,950. I think these prices are reserved for the totally minty or bnib guns from the 1980s. I wouldn't pay these prices for a gun with significant usage. They are not THAT rare!
Police Surplus P7M13. ARMTAC brought in 15 of these at the beginning of 2012 and sold them for $999 each. All were one mag and no box (and mags are a huge issue with the double-stack models, as they are much harder to get). People have successfully scalped them on the EE for $1,500 and possibly as much as $1,800. I personally would not pay over $1,200. The $999 was a good deal, but it is not the same as buying a civy gun with very low miles on it.
P7M13. I bought this 2002-made one for $2,250 bnib with two mags, box, manual and brush. It never made it up on to the EE, but would have sold no problem at that price. This is one of my favorite guns.
This 1980s one (not mine) sold for $1,800 on the EE earlier this year. Fair value for M13's is about $1,500-$2,250 depending on condition and mags/box/brush/manual.
P7M10. Very rare. Probably in the $3,000 to $5,000 range, with most being North of $4,000.
P7K3 and other variants. Fuggedaboutit. Extremely rare and even more desirable.
Ultimately, in the world of P7's, you are dealing with a pistol that is no longer made; has not been made for some time; is very desirable; and is collected by a ton of people. The price of anything is what someone else is willing to pay. Tons people get the urge to try (and usually then buy) a P7, and the prices keep going up!