(Boyfriend + kitchen scissors. Like all first-time amateur coiffeurs, he was painfully tentative at first, wary of snipping even a single strand in case Everything Went Wrong. Twenty minutes later he was merrily wrenching my head from side to side as I bent over the bath, scrubbing my neck with his electric razor.)
Shampoo: Bicarbonate of soda, £2.99/kilo
For short hair, this is a godsend: twenty seconds to rub it in, ten seconds to rinse it out, no need for conditioner. Quite apart from the money, the TIME I have saved!
Cleanser: Apple cider vinegar, £2.50/75cl
Do remember not to spill this on the bathroom floor or you will not be popular.
Telling the industrial-capitalist-patriarchal beauty complex to go fuck itself, even if it's just for one day: Priceless.