I'll add that creating a darkroom need not be an all-at-once thing. You can start doing film developing with a developing tank, a room that's dark (in which to load film into the developing tank), a handful of different chemicals, a thermometer, a few bottles, and a few other odds and ends. It's very helpful to have a sink at which to do the actual processing. As you progress with darkroom activities, you'll probably want to start printing. That takes more equipment -- an enlarger, an easel, trays or drums, more chemistry, etc. It also requires a light-tight space that's big enough to hold all this stuff (or at least the enlarger) -- but in a pinch, you could make do with a space that's just partially light-tight, if you use the darkroom only at night and if you can minimize other household lights, street lights, etc., from lighting up the darkroom. This is the way I worked for a while. I didn't even have a sink in my "alpha-test" darkroom; to wash prints, I had to walk them out to the laundry room. This was a pain, but it let me get my feet wet (not literally, fortunately). I've got prints I made in that makeshift space hanging on my walls, with no plans to take them down.
Overall, the two biggest structural tasks in preparing a darkroom are making it light-tight and in providing plumbing. If you've already got a space that does one or both of those things, then you're halfway there. The plumbing is likely to be the most expensive part of this. I paid $500 to have a utility sink put into my darkroom. This isn't an ideal sink for a darkroom, but it's usable. A dedicated darkroom sink would probably at least double that cost. By comparison, I put up my own drywall for much less -- probably between $100 and $200. Of course, you could save by doing your own plumbing or spend more by paying somebody to light-proof a hopelessly light-leaky space; it's your choice. I was comfortable trying drywall, but not putting in a new sink.
As for buying all the equipment (enlarger, tanks, drums, etc.), if you buy new it'll be expensive. If you buy used, you can probably get everything you need, even including a starting supply of chemistry, for $200 or so. Some people luck out and find everything for free, being thrown away by a newspaper, school, or some other organization that's dismantling its wet darkroom.
