Cheap ways to improve my rough idle and gas mileage?

The first thing I would do is run a compression check to get some idea of what kind of shape this engine is in, then I would check fuel pressure and make sure it's up to specs. You need to check all your vacuum hoses and connections to make sure none are leaking, this can cause both inconsistent idling and running. Once you know that those things are OK, you can start trouble shooting a rough idle, but I suspect your engine had burned valves or something like that.