If you get many posts, i expect they will be quite diverse. The simplest answer is "whatever someone will pay for it".  Along those lines then, it depends on what you want to do with the car. Do you get pleasure out of rebuilding old cars? Do you like to venture into areas you may not be familiar with? If so, you can determine how much enjoyment awaits you as you encounter, and then repair major rust and structural problems. You will learn a lot about rust removal, especially how to use cut off wheels, plasma cutters, mig welders, etc. If you are already set up for this then it should be fun, and you will already have an idea of the labor and costs involved. Do you plan to search for donor parts, or buy repopped metal if available? Sometimes the search for parts is extremely rewarding in itself! And as i'm sure you know, body work and rust repair is usually the most expensive part of restoration, the mechanical stuff is actually straightforward and more reasonable (assuming you can do most of it yourself). And finally, don't expect to recoup  your expenses if you sell your completed project. So, determine what the car is worth to you, after you determine the value of the enjoyment you will get repairing and driving it! BTW, these projects can be A LOT of FUN!