My wife has been bugging me to get an exercise bike for her. Being cheap, I found an old trainer at a thrift store for $10 and an old Diamond Back Sorrento MTB on CL for $20. A few hours work and a few items I had in a parts box became a decent exercise bike. A little adjusting got the saddle height, bar reach/drop and KOPS distances all set to match her road bike. She rides SPDs on her road bike so I put an old pair of Shimano M505s on it and she now happily spins away. The bonus is that I got the MTB fully road worthy so we now have a loaner bike if the need arises.
A lot cheaper than the alternatives we looked at.