The Charge! blog has an article on making cheap trees for your gaming table out of roofing nails and old sponges or foam.
A guy in our gaming group did this many years ago. Tragically, at one point, he leaned over the table, lost his balance and put his hand down to catch himself. His hand landed on one of his own trees, and drove the nail through his palm.
That avenue for cheap trees was a dead end, as far as the rest of us were concerned.