I bought 60L in a small compressed block from tesco's for £1.80!
It's really good stuff but when I went back it had all gone and they haven't restocked.
I think it must have been a health and safety decision because there was a warning on the block about the weight which far exceeded what you expected for the size of the block. In fact I remember struggling to get it into the trolley and asking to have the barcode checked in-situ! If I'd done a risk assessment I would have had it withdrawn from the shelves myself, hopefully to my allotment!
Wish I'd bought more now when I had the chance!