I'm assuming that if the cheapest possible chassis is an issue then the "spec line" isn't.
A lot of people here will tell you that the base car is the smallest expense in this even if you buy a nice 99+, and if you plan to build a nice competitive car that's true. If you just want to bolt on the bare minimum parts per the rules, run used tires that others have given you, etc. so you can get out there and get started, then of course that's not necessarily true.
But, I wouldn't even consider buying one at any price if it is high miles and the underside is a rusty mess. You will spend more time and money snapping off bolts and taking things apart with a torch than you think you have saved on the car.
To be absolutely blunt, if a few hundred or even a grand more for the tub is really an issue then you are better off waiting. Save cash and buy a safe but tired race car that you can freshen up over the course of a year.