The cheap bags are the 3oz bags which are not worth it, you can literally see through them, decent long handle retail quality cotton shoppers are about 5oz. 10-12oz bags tend to be the ones with a gusset. These still print really nice but as SMO says, they are about £3.00 including VAT.Depends if you want cheap nasty bags (promo bags) or decent 12oz cotton bags
If you think this is an avenue to look down then I strongly recommend you get some samples from Direct Trade. I think you will be pretty impressed at the quality, price and how well they print with a sheet of TTC.