Please tell me why and an estimate for the price would be greatly appreciated....thanks

Ply wood. duh

Nothing beats unpainted plywood. It has a nice spring to it when you land on it, much better than the wrist snapping thud of concrete.If you paint it it will be loose and slidey (so dont). You should use marine ply if you want it to last but 1/2 inch (12mm) marine ply will cost upwards of $50 a sheet.
Expect to spend many hundreds of dollars in materials to build a half-pipe unless you can get your hands on cheap (borrowed) materials.

l thought this would have been answered by now.

Masonite is very flexible, durable & a good price, for what u get. It is moderately smooth & will take a coat of paint to really make it nice, both on the eye & for the skater.

Now if you're really serious about ur ramp, Skatelite is freakin' awesome. It is the material they use for professional ramps & exhibitions. It is tough as can be, eay to work with & is very durable - more so than masonite.

Choose ur budget.

Skatelite is the best but it is expensive (around $150 per sheet.) Massonite is also good because it smooth. l would use the Massonite and just put a tarp over your ramp when it rains.

idk