The main cost is their living quarters (coop and run, unless you will freerange them in which case you only need the coop but you will lose chickens to predators).
Often you can adapt some other structure to make a coop (playhouse, shed, packing crate, etc); if not, you may at least be able to score some free or cheap materials if you scrounge around. It needs to be well and truly rainproof and predatorproof though, with predatorproof (screened) ventilation.
For a run, you are well-advised to use something sturdier than chickenwire, which despite the name is only *mostly* chickenproof and not really at all predatorproof. Price it, and scrounge around. It will need strong posts as well, preferably pressure-treated lumber if the posts will be set into the ground.
Feeders, waterers, nestboxes, and roost can be homemade from scrounged materials for essentially free if necessary, although personally I think a $4 plastic 1-gallon waterer from the store is a good investment (larger or sturdier is an even better, but slightly bigger, investment).
Feed is somewheres around $10 per 50 lb bag, which would last 6 chickens something like a month. Shavings (for bedding) are around $5 per bag; how mcuh you use depends on your setup, but for half a dozen chickens or so I'd be surprised if you would need more than a bag a month and there are lots of ways of using less than that.
Does that help? Good luck,
Pat