I bought a masta one for £17 from Go outdoors. I have not used it yet but it looks quite well made.
I also have a shires tempest one that I bought for £35. This one is fab, I have had it for a few years on numerous horses (including a serial rug trasher!) and its still going strong.
Have a couple of cheap derby house ones that have done our two vandals for years! Just their basic cheapest fly rug, it looks exactly the same as the Hy one so that should be a good bet too http://www.equestrianclearance.com/_battles/hy_guardian_fly_rug_fly_mask/index.html
Either the Hy or Derby house one can be got a tonne cheaper if you keep an eye out for offers!