I just worked mine out; I use Waitrose dishwasher liquid. It's £3.67 for 1l, and I use 10ml per wash on average (sometimes a bit more, sometimes a bit less), so that's 3.67p per wash.
An added complication is that I never pay £3.67 for a bottle because I only ever buy things like that when they're on offer, and I have 3 bottles in stock currently from past offers.