A 1-meter scale trebuchet design is available at WikiHow.com, and a tabletop version is available at StormTheCastle.com. The materials for the large version cost less than $15, and the finished trebuchet can throw a croquet ball up to 30 meters. The materials include four 8-foot lengths of two-by-two wood, 1 foot of welded chain, drywall screws and two metal T-plates.
Build the base and side supports using the lengths of wood, adding plywood to create a base. Attach the chain to the throwing arm and secure the throwing arm to the frame using a metal conduit. Use the chain to attach a small bucket so that rocks can be added as ballast. Fix a string and a sling to the other end of the throwing arm, and make a trigger using a bent piece of coat hanger. The number of ballast that is required depends on what is being thrown. It is necessary to experiment with different weights to find the ideal balance.
A design for a much smaller trebuchet is available from StormTheCastle.com. The trebuchet is around 12 inches tall and is designed for use on a tabletop. It is also made of wood, and features a counterweight made from a small rock or several rolls of pennies. The small trebuchet should take around two hours to construct.