Bjorn and I have just finished off reviewing a couple good flys at the cheaper end of the spectrum.
Bjorn's, the Oz Trail Hiker Fly, is heavier but very tough, and sells for a remarkable $50:
http://fatcanyoners.org/bush-guide/oz-trail-hikers-fly/Mine, the Aussie-made Terra Rosa Gear Exploring Tarp, is a little bit more expensive ($90) but still probably the cheapest ultra-light fly you can buy:
http://fatcanyoners.org/bush-guide/terr ... ring-tarp/For overnight / multi-day canyoning and bushwalking in the Blue Mountains a fly is almost always up to the job, and generally means less weight and bulk in your pack than a tent (plus a lower price tag) so perfect for students / tight-arses looking to buy their own gear.
I was particularly excited by the use of Tyvek in the Terra Rosa Gear fly. It is a great option for getting ultra-light gear that is lighter than silnylon and HUGELY cheaper than cuben fibre.