If cost is a concern, there is no way you can use lexan or fiberglass.
The most cost convenient route would be to use the plans here:
http://www.mikenchell.com/images/GenericBenroyPlans.pdf, but make the trailer 4' wide instead of 5' wide.
You would do construction of the outer walls using 1/8" luan plywood. It is about $10 a 4'x8' sheet at Home Depot (or similar), and you will need at least 4 sheets for the Benroy.
The simplest construction (given little knowledge) would be to:
1. Build the chassis from a kit (such as the Harbor Freight or Tractor Supply Company kit)
2. Mount a thick (1/2" thick is more than adequate) piece of 4'x8' plywood to the trailer using carriage bolts
3. Build a frame out of 1"x4" or 1"x3" studs (both are usually a bit over $1 each at home depot -- you'll probably need quite a few to build the frame)
4. "Skin" the frame with luan plywood, and use a router to follow the curves of the frame
5. Seal the exterior with CPES, then some sort of varnish
If you look around the forums and at the plans long enough, you'll start to figure out how everything works, and how difficult (or easy!) a teardrop can be.
The key is to PLAN PLAN and PLAN some more before diving in. You want to make sure you get everything right the first time!
