Go to Boots. You can buy stuff and do it yourself. It's fine. I get stuff that's supposed to be semi permanent, get one without ammonia as it smells vile. It fades slowly so when it starts growing out doesn't look too startling.
oh god no dont do it yourself! cost really depends on what hairdresser you go to and how much you have done, you can have a few bits at the front, half your head or full head. do you have a regular hairdresser? if yes go in and have a chat about what you want and cost, if not ring round for prices, they can vary loads