There are a couple of ways to look at it.
If this is the only boat around and you really want to get something fast, you should definitely negotiate the price down a bit and buy it. A 1981 Prindle isn't going to fetch much anyway, so just decide what you're comfortable spending for a boat that might have significant problems and offer that. If you get it at a cheap price, sail it and repair it (or not) and enjoy it.
The repair really isn't difficult . . . . .and when you're paying for something so depreciated in value, does it really matter how well it looks? You'll spend $50 and 4 hours of your time to do the repair. If your first Do-It-Yourself fiberglass repair looks crappy, so what?.
If it gets you into a cheap boat immediately and you get some fun out of it, that's all you need. In two years, you can sell to some other guy for not much less than you bought it.
The absolute worst case scenario is the hull breaks off and you look on the used parts market for a spare hull . .. or . . you sell the remaining parts of your boat on eBay to recoup some of the losses.
The other way to look at it depends on the availability of other boats in the area. 1980's era boats aren't going to cost a lot (with or without soft spots).
I bought my Mystere 6.0 and it had a small soft spot on the deck of the hull near the rear cross beam. It worsened considerably through my first year (you can imagine my dissappointment), so I did the repair suggested above. It worked a little bit, but the Mystere deck construction in that area is a little different due to it's sub-decks. After contacting the head Mystere guy, he suggested laying down a few extra layer of fiberglass on the top of the soft spot (simple . . .apply epoxy, lay down some glass, apply more epoxy, lay down another layer of glass . . . . let it harden, sand it down . . .if it's still soft, repeat). That's what I did. It looks aweful because I rushed the job, but it's as solid as a rock. If I was so inclined, I would spend a day sanding it down smooth and adding some gel-coat. But that's a project for another day (year).
I guess my long winded point is that a soft spot isn't the end of the world.
