Being the owner of a 1975 Carrera, numbers matching car, all in perfect as can be shape - minimal rust (which has been repaired) I can not see these cars being worth more than $25,000 to $30,000.
Modified they are valued (to me) no different than a 1975 911S, $7,000. Put the numbers matching motor/trans, without body modifications and rust - in good running shape, gets to that $25,000 to $30,000 range.
In defense of the 1974 and 1975 Carreras, I will say this. Rust killed alot of them. Where the rust didn't, the 2.7 without cooling did. Again, we are talking about NUMBERS matching cars.
I am going out on a limb here, but I would bet there are no more than 50 actual numbers matching 1974/75 Carreras around. They only made a few thousand between the two years. Does that make them desirable? Only to collectors. Is there anything special about them, performance wise? No. With less than 100 and more than 50 with the ORIGINAL motors and trans - does this make them readily available? No.
Go find a correct one - clean, numbers matching and try to spend less than $25,000. You won't.
Are there better cars for the same price or LESS? Indeed. I will take it to the extreme here - a 1997 993 is a faster, more refined car than a 1975 Carrera, but can always be found for less.
I would not trade my '75 for a 993. Never. I can find a 993 any day of any year. There are alot of cars you can find any day of any year. 1974 and 1975 Carrera's are not those cars. That makes them worth something to someone.
