My DH built our boys a couple of ziplines a few years ago. I would be very leary of paying $60 for a zipline - you do not want your child on a zip line that is made of very thin and cheap wire. You need a thick, heavy duty wire on your zip line, for your child's safety and thick wire is not cheap.
My DH paid more than your budget doing it himself - but he made sure to use good quality products for the zip line. DH's zip lines have been used for years, and are still safe for our now teenagers to have fun with.
Just FYI - a local Atlanta woman fell into a river when the zip line she was riding on broke this summer. She ended up getting flesh eating bacteria and losing her legs and hands. Splurge for safe wire!