I know there are 'competing materials' to Skatelite. And as 'jb' suggests, I'd sandwich at least 3 1/4" pieces of plywood. I suggest the middle piece be a cross-laminate (90 degrees) to the top and bottom layers. Of course, as 'Dave' says, it would have to be weatherized.
You may want to shop around to see what else is out there. I've provided a list of 'hits' from My Favorite WEB Search Engine. There's a mix of hits that include the following: Parks, Construction Material(s), Plans, and more. Perhaps you can contact some of these 'places' to see what they used or how to get a discount. I'm sure a product like Skatelite is much better than plywood and a lot safer. Besides, if the individual that's 'using' the ramp is planning on going into competition, it's going to be night vs. day when you go from wood to something like Skatelite. I know it's a commitment but there's nothing like practicing on 'the real stuff'.