Being involved with just about any type of outdoor ramp/bowl, I recommend not too be concerned with how much money you spend on it. Save and be prepared to spend time and money. Look at it like its never finished, and be ready for maintenance. Top layer should be sustainable. Wether you go with skatelite, steel or concrete, you need the riding surface to last. Concrete is probably the cheapest and will involve less maintenance, but a little more difficult to build. Skatelite last pretty long time (10+ years), same with well painted steel if taken care of. You will need to pull up those every few years to fix soft and water damaged plies. Take into factor the weather where you live, the exposure of the ramp to the elements and what you like to skate to determine what you end up building. Build it off the ground to allow air to flow under, slope the flat and the decks a bit to allow water to flow off, and try not to let water pool and sit on the flat, the decks and especially behind the coping.
Ive found tarp covering never works, you ended up forgetting, it blows off and becomes a major pain in the ass to deal with. From your measurements, Id budget at least $2500 to have a good, fun, long lasting ramp. Good luck and post progress photos if you go for it.