Yes, me too, but miffed about that. Seems the supermarkets have cottoned on. About 6months ago, my mate was buying it for £8 for 15L, now it's nearly doubled! However, assume you're doing a 60(oil)/40(diesel) mix, you still save money.
Take diesel at 1.25L and the MkIV tank is 50L. so 1.25x50 = £62.50 a tank of diesel.
60/40 would be a bit cheaper -
diesel = 20L x 1.25 = £25
oil = 30L x 1 = £30
Which is collectively £55. So you save £7.50 on a tank if using 60/40.
I only looked in Tesco though, not sure how much cooking oil would be in big wholesale places such as Costco? I know someone who goes there regularly though, so will ask. Asda/Morrissons are generally cheaper than Tesco too, so they may be worth a go. I still think it's worth it, I'll save £15 a month.
There is the option of getting used oil from somewhere, but I'm not sure about the filtration/cleaning process. Also, I've a feeling that oil suppliers are demanding the oil they supply is returned when used. May be worth investigating though.