You don't have to buy the latest and greatest to have a hell of a lot of fun.
Since all manufactures have shifted to 4stroke, there are tons of 2stroke trial bikes for sale. A 5-8 year old 2stk is in the $800-1k range. They weigh about 150 lbs and can easily be carried most anywhere. A $150 set of tires will last a new rider many seasons.
The biggest objection is usually, "It don't got no seat!". Yep that is true, but dirt track bikes "got not brakes". The banana frame is to make is really easy to balance, hop, jump, etc. I Utah we fill in the gap with a tank/seat combo; and that makes a phenomenal trail bike.
I don't understand the pedal bike v. motor bike controversy. Our trails will be marked as single track or otherwise. The Stansbury trail is a 25 mile single track that is shared by motor/pedal bikes and horses. When horses approach, we stop and shutdown until the pass. Everyone talks to everyone.
Quads have their own trails, which usually are more like the old jeep trails.
For two of my favorites, search "slick rock trail, Moab" and "5 miles of Hell, Moab". There is also a 200 mile trail system in SW Utah called, "Piaute Trails".
Here's a good place to start:
http://www.riderdestinations.com/utah-atv-trails/
I probably sound like a commercial, but I have ridden most of these and they are all good cheap fun. Camping is available on everyone of them.
Now go get dirty,
Cheers,