I think the soft-window targas are highly desirable for a small portion of the early 911 community. The early 911 community is already pretty small, so an even smaller portion of them are interested, but those that are can be fanatically interested. So, it seems the prices are all over the map depending on if you can catch one of their eyes. Rust free and original seem to be the most important factors. As a worst case scenerio, though, I did see a 67S roller softie with no engine or trans on Samba last week for $16K. That's probably a bit high, but should at least give you a minimum value as a start point. Since it has a motor and trans, then the value is certainly greater than $15K or so. If it's a numbers matching motor and trans, I've seen really poor and rusty S targas go for $30K. You could post the motor and trans numbers. If they are basically correct for the year and car type, then it's likely numbers matching, or you could order a CoA from Porsche. Costs $100, but you'll make that back in spades if you can document the engine is numbers matching.
As to numbers, there was a thread on the early911S registry last week asking how many softies were left. There are more than 6. On the registry, there are 11 registered 67S targas, and I'm aware of at least 5 68S softies in the US, one of them is mine
. So, they are very rare, maybe 100-500 left, but not single digits rare.
Best of luck on your sale...but with a car that rare and interesting, I'd consider driving it for another decade or two. As long as you don't park it on the bottom of the Atlantic, the value will still be high and then you can sell it.