Here are some ways to create a 501st costume on a budget. Comments are from a discussion from members in the NEG forum.
IC or “technician” in black or gray estimate $100:
Costume components can be used for future ones (TIE, AT-AT, AT-ST, Biker Scout, Officer, Death Star Trooper, Imperial Gunner).
Costumebay/Costumebase is an ebay vendor providing Star Wars costumes for a low cost. Quality varies and most costumes will require adjustment before getting garrison approval, but folks have been happy with Imperial Technician costumes from this vendor.
A self-made imperial office costume can be much less expensive than an armored 501st costume.
JoAnn Fabric offers 40% and 50% off coupons to mailing list members.
Snowtrooper: estimate $350-$500 range:
E-11 only $30 through Disney (use “shop at home” service through parks).
Tusken Raider: estimate $150