I bought the Gardman 2m x 3m which sells for about £80 (though I found mine on clearance for £40) it is probably the best of a bad lot and the frame will be useful for some years but the cover is ***** and I will be lucky if I can repair it for another season. It is buried about 15cm down and earthed up inside with raised beds so it is pretty well anchored or it would be gone with the wind by now.
There is a lot on the net about DIY polytunnels but even if you are very skilled, it will still cost you £150 for the hoops and polythene cover:
http://www.overthegardengate.net/UserPages/pp_view.asp?FName=Polytunnel&Page=1But if you can, that is the way to go. There are posts on here about what people have managed to build, quite ingenious but they have all required some skill, a decent cordless drill and materials money. Read up on stuff and start saving up is my advice and what I am doing.