Without seeing pics or knowing more about the details and or location and how much rust is hiding underneath, maybe $4K-$9k? Maybe far less, maybe a little more. Too hard to be exact with little info. Sorry.
As mentioned, need lots of info. You need to crawl around the car looking for rust in and under everything. Last car I bought, I spent about 1 hour just looking for rust. The area you are in and how the car is equipped also determine the value. In the end, it is worth whatever you are willing to pay. If you are happy, then it is done. If you are trying to turn a buck, it is a lot harder to price. Let us know more.