You rarely see these fantastic quality bin liners for a decent price. a 10 pack is £2.39 + delivery with Amazon, and 30 pack is £9.52 at Amazon. So to get these for the equivalent of £1.84 online or £1.29 instore for a pack of 10 is very good in my opinion (although I'm sure the naysayers will vote cold because Asda own brand bin liners are 1p each!)
Robert Dyas currently have Brabantia bin liners on a 3 for 2 offer, and also a 15% off code (JULY15).
Packs of 30 are £7.49 each, so 90 are £14.98 on the 3 for 2 offer. Taking the 15% off brings it down to £11.61 (as the 15% is deducted before the 3 for 2 discount). Then add on £4.99 for delivery. Instore it should hopefully come to the £11.61.
Don't forget 5% Quidco.
Also, if you spend £50 you get free delivery (but that would be a heck of a lot of bin liners!)