Soya
Cheapest I've found: Tesco unsweetened Everyday Value longlife at 59p
Asda smartprice unsweetened longlife at 59p
Others to note: Morrisons do a sweetened and unsweetened longlife for 60p each.
Oat
Cheapest I've found: Asda's new own brand longlife oat milk at £1
Others to note: Alpro frequently have offers on oat milk, for example, when I made this post, Morrisons, Sainsburys and Tesco had Alpro longlife oat milk for £1.
Almond
Cheapest I've found: Sainsburys longlife (both sweetened and unsweetened) at £1 permanently.
Asda have a variety of fresh and longlife, sweetened and unsweetened own brand almond milk for £1
Others to note: Alpro is again often on offer, with Tesco selling the Alpro roasted almond longlife milk for £1 as I type this!
Rice
Cheapest I've found: There appears to surprisingly be no 'cheap' rice milk. However as I type this, Rice Dream is on offer for £1 in Morrisons and Alpro is £3 for 3 there too.
Coconut
Cheapest I've found: Asda chilled free from coconut milk at 97p
Others to note: Tesco currently have Alpro longlife coconut milk for £1, (as do Sainsburys and Morrisons) and Koko dairy free chilled at £2 for 2. Morrisons also have Coconut dream for £1 and appear to sell fresh Alpro coconut milk for £1 all the time (not an offer price!).
Hope this helps a little. If anyone chooses to say 'veganism is so expensive!' feel free to point this blog at them, tapping the 59p soya milk, then staple a print out to their forehead. No don't. Please don't. ;-)