It's sort of like asking "how much is that" when you look at a fancy car...if you have to ask, you can't afford it.
Having said that there are three questions to help you get your answer:
1. Is the jeep structurally sound given the rust situation? Best not to buy it if you can't determine that.
2. What do you want the jeep to look like once repaired and how much can you take on yourself?
3. What other repairs/improvements do you plan to do? If you are going to lift it and replace suspension components, below is a list of the nuts and bolts that might have to be cut out, will strip, break, or weld nuts that will break when you're working on the suspension of even a moderately rusty cherokee....
ALL OF THEM.