Is keeping chickens in the UK cheap?

i want to have 15-20 chickens eventually, at the moment i have only got 5. but when i move too another house, how much will it cost to build a large run, buy a house and all of the chickens, i will probably start out with 10 crossbreeds because there cheap, and then hatch some purebred chicks in an incubator and introduce them to the flock and expand that way. but how much does it cost to start it, and then keep it. and how many poultry birds do you have?