I believe Pinto prices are going up simply due to the laws of supply and demand. If you look at the recent influx of people here on this site, it is a good barometer of how interest continues to swell towards the Pinto.
Then, add in the fact that most people (unlike several of us crazy folks

) have neither the time, facilities, or fortitude to rebuild a Pinto from the ground up, the search for complete cars drives demand - and prices, because there are not a LOT to choose from, unlike the plethora of Camaros, Cudas, Mustangs, Corvettes, and other "has-ran" vehicles available.
In the meantime, until the Pinto collector market grows and is satiated, and demand rises beyond availability, those Pintos sitting in fields, barns, and junkyards SHOULD remain inexpensive to those wanting to build. Case in point - I bought a running 1957 two door hardtop, solid, a builder, back in 1983 out of California, driving it out of the guy's driveway - for $800. No joke. Today, with MANY THOUSANDS restored, those wanting to build similar cars that are sitting in fields are likely to pay $5k - even without motors and trannys - because the supply has dried up.
Moral? Get 'em now - "Be like Fred". As demand grows, supplies fall, and with so few out there, you'll continue to see prices go up. What I am looking forward to is the point where demand allows for an economical aftermarket need, so repro parts can be built. Back in 1983, 1857 Chevy repro parts
were available - today you can buy a WHOLE BODY. This of course will never happen with the Pinto, but there is definitely room for repro parts as parts cars are depleted.
Chris