Trommels are expensive because they are usually motor powered. A homemade hand-cranked trommel can save you a lot of money.
A steel drum is the most durable and affordable object you can use as the basis for a trommel. Find steel drums online or at a dump. Thoroughly clean the drum before using it. You will need a lever to serve as a hand crank, a screen and a stand to support the drum.
To assemble the trommel, cut the drum into thirds. Replace the middle third of the drum with the screen. Attach 6-inch metal rods to either end of the drum. Attach a lever the end of one pole to act as a crank. Build a stand with a rectangular frame large enough to surround the drum with the poles resting on the frame.
Use your trommel by loading the objects you wish to sort and cranking the trommel until unwanted particulates fall through the screen. Use water to wash out dirt.