Too much! I bought two bikes for 400, one complete, the other missing seat, side covers and airbox. I also bought another about a month ago for about 200. All have nice tanks, bit of surface rust inside that electrolyis will take care of. If you're like me, recently I was in a bit of a panic cuz my first bike has rusted fork tubes, was about to pay too much for new ones with a whole bike to finish. Then I dug around, even here on Vancouver Island where opportunities are few usually, and bingo. Both bikes of the newer purchase have nice forks and one has an original exhaust that looks pretty darned nice.
I agree, that tank has a circle for a reason, right by the petcock and a scum line on the bottom like it's been in a puddle for a few years. Keep looking locally. If you just need a temporary fix, there's all sorts of things you could adapt. Are there no bike wreckers in your area?