Have you ever packed a parachute, rigged a bungee drop, or set-up a Hang-glider? Zip-lines are similar in that only someone who is truly proficient and/or certified should do it. The equipment itself is over a $1000 for a basic 50 foot zip-line set-up (not including tower platforms).
get really thick and long string, tie it to 2 trees,then use a ladder to reach up the tree, last get a hanger to go down the rope. warning u might hit the other tree though lol...i wouldnt do this cuz u could get hurt but if your desperate, then use this.