I'll be a contrarian. We have a treadmill we got for $25 from a neighbor who was having a yard sale (he even helped carry it into our basement). It's been worth every penny. I don't necessarily use it consistently (when it's nice out I'd rather run outside), but I stay at home with two small childen so can't go workout whenever I want without figuring out what to do with them, and it's snowy/icy here a chunk of the year, so it's nice to be able to run inside. My SO can use it, too, and get a run in before work when it's dark out without worrying about getting hit by a car. It's not the greatest for long runs because it can get boring, but it's better than nothing. And it's worked well so far for HIIT.
My suggestion is to keep looking through Craigslist/garage sales. Also, if you're worried about it turning into a clothes rack, then have her figure out how many months of workouts she'd want to get out of it in order to feel like you've gotten a return on your investment. If you have a gym membership now, then maybe it can replace that cost for X months. After that point, any workouts you get from it are gravy.