Greetings and welcome to the forum.
One really cheap costume is the Rebel Fleet Trooper. Gray khakis, blue suit shirt, black vest... They're all pretty cheap. Helmets are fairly cheap, too, and I've seen holsters that aren't too expensive. The blaster can be a simple Hasbro blaster that's painted black and silver. The most expensive thing would probably be the boots.
The X-wing pilot costume usually isn't too bad, either, and you can buy almost all of the parts for it already finished. It's even cheaper if you don't get a helmet (which is optional as long as you have belt flares or the accurate belt buckle).
