Tesco Dishwasher Cleaner liquid, Two Bottle multi-pack (2x250ml bottles) is reduced to £2, apparently from the usual price of £4.15. So each use costs only £1.
Finish also has a 3 for 2 deal at tesco, but that works out more expensive as it's only for a single pack (over £2 per use).
Haven't used this yet, but can't really lose out at £1 for each cleaning cycle. Instructions suggest cleaning the dishwasher with this once a month - which might be a bit of an overkill and unnecessary energy/ water use. I usually run the cleaning cycle once every 2-3 months but I clean the filters under the tap more regularly.