skatelite is VERY expensive, usually...
if you've never made a ramp (and even if you have..) I would suggest using plywood for the surface. paint it to protect it from the elements. DO NOT USE wood sealer. it usually splinters after time and is the worst surface to fall on. paint works better from my experience.
if built correctly, the surface shouldn't warp. also, in two years time, you may no longer ride or want to rebuild the ramp so it's up to par with your skills. (most people build a smaller ramp, then get used to it and rebuild something new and exciting.)
I would suggest using 3/4 ply or 5/8 ply for the sides, 2x4's for the stretchers between the sides and a couple layers of 3/8 or 1/2 for the surface. depending on how wide you make it, you may or may not want coping.