If you're after serious shellfish, steer well clear of the foldy ones. I tried one, didn't last 5 minutes. the net is not much heavier than the stuff they make the kids fishing nets out of from the bucket and spade shops, even small shore crabs/velevets will rip straight through it.
the other problem is the size of the eye, they're not very big so the post will fill with loads of small velvets, and undersized brown crabs
best bet is go find a working pot boat and ask to buy a couple. I got 3 for a tenner last year. they chap didn't want anything at first as the pots were in his opinion on their last legs for commercial gauling but for me they've been champion, couple of teeny holes to fill up with twine and that was it.
go for crab pots rather than true lobster prawn pots. the eyes are bigger so large brown crabs can get in and there's stacks more of them than lobsters around, but you still stand the chance of the odd lobby now and then.
they're not that expensive new though, and a couple of new ones will last a lifetime, unless you plan on going out everyday
http://www.gaelforcemarine.co.uk/ProductsMenu.aspx?cat_id=4478 27 quid apiece plus 7 quid for postage