The price of boats with soft hulls should be the equivalent of the major GOOD used parts on the boat: mast, sail, tramp, rudders-and-castings. For a late 70s boat that has been modestly used in fresh water, chances are the sails moderately blown out and chaffed, with some stitching broken and pulled out; rudder castings are half worn and a bit fussy; tramp is about shot; wires are questionable; if the mast has a comp tip- the tip may have age problems.
Now, if the mast is all aluminum and hasn't been dented or bent, it is probably in good shape. A good mast is worth $300.

So, the above typical freshwater boat without a trailer is worth at best $300. For recreational use, worn sails not needing repairs is ok. Add up the cost of the needed replacement parts and epoxy, and YOUR labor. I'd guess you would average out to about a $1000 boat, AFTER fixing it up.

For $1000, you should be able to find a boat without problems.