dude, dont ever use gasoline... Gasoline will literally eat away at the walls of your PVC, and the chemical cement used to fasten the connecting chamber. If you use gasoline in a spud cannon with a leak you will most likely get burnt very bad.
In my opinion the best fuel is butane or propane.
Hairspray is great and very easy to use but it will leave some nasty black charred crap on the inside of the chamber and/or the threads to the back of it. Not to mention all of the ingredients in hairspray are not flammable and you will need to wipe out your chamber or let it drip out. If you want a cheap aerosol can use right guard deodorant or starting fluid. they wont leave much of a mess inside the chamber, right guard smells good too
I use a propane tank with simple torch adapter to let the propane out of a tank. small bottles can be purchased for hand held use and no special riggings are needed to use propane.
the cheapest fastest way is an aerosol can. no doubt