A couple pros, and a couple cons.
I'll start with the pros:
Go for it dude! Why would it suck to have a big ramp? You can always speed check, but it's way harder to push for more speed. A big ramp just means you can have any feature without having to worry about speed.
The bigger the ramp, the bigger and more creative you can make your features.
Here are some cons:
You mentioned that your ramp was going to be 45º. Do you have any idea how steep 45º actually is?!? Some of the world's steepest slopes aren't much steeper than 50º! A ramp that steep would make sense if it was a small ramp, because you'd need as much speed as possible. But you should definitely mellow out the ramp a little bit, because a 28' drop of 45º is going to have you hauling fucking ass by the bottom!
As for costs, sorry to break it to you bro but there's no way you're going to be spending $150 on this thing. When you totaled your costs, did you take into account any angular bracings you're going to have to include? Did you take into account the cost of screws? (Use screws for something like this). Did you take into account the actual cost of the ramp itself? (you can't just use 2x4 to make a ramp. You'll probably want some type of sheet wood (plywood would be ideal)). Also, if you're running a 20' high structure, you're going to want something thicker than 2x4. I'd suggest 4x4 at least, maybe even something in the range of 6" or even 8"...? When I built a mini-ramp that I used for small jibs around town, I built a tower 5' high, with a footprint of 2' by 2'. The ramp was an 8' long by 2' wide sheet of plywood, reinforced with 2x4 bracings. The tower was supported by 4 4x4 beams, with 2x4 angular braces. The entire thing run in at about $140.00. Admittedly, I bought decent quality outdoor wood, with anticipation of it lasting a couple seasons. You, however, are thinking of building something at least 5 times as big. For simplicity, let's say in total my ramp cost me $150. This would mean that your ramp is going to cost you upwards $750.
Anyway, I'd still tell you to go for it, but you have to take more into consideration before you build it. Looking at your blueprint, you need to do a lot more to make it safe and sturdy. Take it to somebody at a hardware store, show them your work, and have them draw something up for you, or you're going to get hurt.
But good luck! Have fun.
