I just put a darkroom in my basement. Well I basically turned the whole basement into the darkroom, I didn't build new walls or anything. I just covered the windows, put weatherstripping on the door at the top of the steps to block the light, moved some workbenches from the garage down, and changed out the sink, put up a safelight, etc... It's not an ideal situation due to the washer and dryer so dust may be an issue, but it's doable for now and I can always figure out a better solution later.
Some rough estimates of what I've spent:
enlarger n misc stuff lot (
ebay, $50)
digital timer (
ebay, $60)
black duct tape for windows (over top of garbage bags) - $3
weatherstripping for door - $2, $3, then $5, took a couple tried to find one that worked right
safelight - $20
misc trays n stuff on ebay - $35 (included funnels, papersafe, chem bottles, 5 trays, etc)
print washer - $20?
grain focuser - $10, need a diff one, I can't use this one
6x6 neg carrier - $2.21, god I love ebay
timer for film - $16.50
2 more trays and hypochek - $12, just bought that today at local shop
filters and graduates - 25ish
80mm enlarging lens - $57, altho the bastard hasn't shipped or contacted me yet

stuff I just ordered from
Adorama - paper cutter, temp regulator thingy, a hose, gloves and some film - $116
I think that's it, except for chems and paper...
You could probably get some of the stuff cheaper, because I got some stuff new that I didn't need to, but then I think I got a great deal on my enlarger, so it all works out in my mind...
Good luck with whatever you decide to do!