A good forest is eventually a necessity for most RPG settings. Unfortunately in the hobby world trees are not cheap and as such building a forest is not necessarily cost effective for everyone.
I put off building my forest for that exact reason. I could not justify spending the money per tree and I wasn't ready to try and make my own.
Then I was out Christmas shopping one day and stumbled across these in our local Reject Shop (cheap shop). This was enough for me to build the concept in my head and I quickly searched the rest of the cheap shops in our area and found another tree design that i licked. These are just fake plastic trees. They we're between $8-$18 each. I picked up enough to cover an area large enough for an encounter.
The first thing i did was break the trees away from the pots. They were just filled with foam and glue so it wasn't difficult.
Then I based some Hirst Arts cavern tiles (Cavern Floor Mold #281) onto my foam board base. My dungeon is based on a 5x5 tile-set so i did this but left the middle tile out. I drilled a hole in the foam, and pushed the trunk of the tree into the hole.
Remove the tree and paint the base. Once the paint is dry squirt some glue into the hole, replace the tree and wait for it to dry.
These were simple. A lot cheaper than most trees I have looked at purchasing and overall very pleasing on the tabletop.