If you search for a while, and are willing/able to do some serious rehab work, you can find a good mixte at a reasonable cost.

I started with this Centurion frameset, off Charlotte C/L for $20. I added parts from a donor bike: crankset, bottom bracket, wheelset, freewheel, chain, derailleurs and shifters. Even installed decent 700c wheels on it.

In my area, framesets tend to go cheap on C/L. Buyers want complete bikes, ready to ride. This one sat on C/L for a couple of days (I do not live in Charlotte, so it was a bit of a drive).