This thing cost around $175 in lumber, bolts, nails, screws, and such. It is built entirely out of pressure treated lumber, using 4x4's for the posts, 2x6's for the load-bearing header beams, and then 2x4's for all the cross supports, ladder, and joists. The decking was made from 6" wide decking planks.
The only probem with the platform was that we built it too high. After building it we tried it out only to find that when weight was put on the zip-line it dipped so low that you'd hit the end of the platform. So, we realized that we either had to raise the zip-line or get a bunch of friends over to lift the platform so that we could cut off about 2' on each leg. Raising the zip-line seemed like a better idea, and it was.