Think my cheapest has been my (back to back brews of a) pilgrim smash.
10kg of grain - about ã8 when bought in bulk
100g of hops - ã2.20
1 pack of yeast ã2.35 (reused)
ã12.55 for 80 bottles, just over 15p per bottle.
Most expensive has been where I have added extras like cacao...