Remember that your machine might have to operate up to 50 times at the show. The cost of replacing the consumables is not counted against your bill of materials cost, but the cost of the first instance of the consumable is. Make your robot eye-catching. This need not be complex. For example, something hitting a switch which causes the computer to trigger some other kind of action makes a fine sensor-based robot.
Successful robots have no correlation to the amount of money spent.
Many excellent robots have been built by spending just a few dollars. You would be amazed what you can find in a dumpster or by asking for discards in a store. Build with materials and processes that you are familiar with. The keys to this project are simplicity, aiming for what you can actually build, and keeping costs low. Effects which rely on sound or music are problematic.