I used to be fan of Testors airbrush thinner. I used it only for the actual thinning of paint in the airbrush, not for cleanup. The cost of the thinner to do a kit was negligible compared to the price of the kit and other things like the PE and other accessories. I felt there was some difference compared to hardware store thinners.
Testors airbrush thinner has been very hard to find lately. Don't know if they are discontinuing it or not. So, I had to experiment, and found that the difference seemed to be that I had to increase the thinner amount a little when using hardware store thinner. So I will now be using the hardware store stuff- not for cost reasons- again, the amount of thinner used in airbrushing doesn't cost that much, but of necessity.
One big caveat, however. Be careful which hardware store "thinner" you buy. There are cheap thinners sold these days that are milky white rather than clear. These have a lot of crud in them. I hold out for real "mineral spirits" or turpentine, rather than something just labeled paint thinner. These two specific solvents are more expensive than the generic paint thinner, so I am probably not saving any money compared to the cost of the old Testors Airbrush Thinner.
Also, I have committed myself to learning how to use acrylics. I can see the handwriting on the wall- solvent-based paints are disappearing :-( I frequently do subjects that require glossy finishes, which I find a real challenge with acrylics.