once you've spent some time searching, X's in good condition can be found. it took me a few months, but after a while i found half a dozen x1/9's in my area (adelaide australia) that i would call "excellent" in condition. this, considering i see only 1 or 2 on the road per month.
i'm talking aussie dollars, so you'll have to do the conversions, but a good stock example can be bought for between $5000 and $10000, depending on year and options and condition and basically what the seller thinks it's werth.
luckily you don't have to spend much at all on suspension and whatnot, the car will probly take a lot more than you can throw at it. but for an uno turbo (the lump of choice of many Xer's, me, i've got a twin cam) it'll be a few grand, and then a few more grand on installation and odds and ends.
overall, i'd figure on spending $20,000 to make a car you're totally happy with. i've spent almost $10,000 on mine (twin cam conversion done by PO) including buying the thing, and if i can scratch together another $5k to throw at it, it'll be ready to race.