If you are trying to keep costs down you can always take a thin gauge piece of silver like 20 or 22 and solder it to a piece of nickel as a backer. This will give the feel and heft of a nice chunk of metal but make material costs considerably less. A large amount of the trophy buckles out there are done this way...the ones for smaller rodeos are mainly made in base metals exclusively (nickel with brass and copper overlays).
Stick to solid materials for value if nothing else let alone waiting your time being stingy,it never works out very well. If it is a nice buckle, I'd use 14 ga. solid sterling for a base. ,16 ga. for an average buckle. If it has a lot of shape like a ranger style, 18 will be fine with rope or an added border.