There’s a bell curve. If you burn out too quick you’re not gonna get nearly as much cardio, and the torque required to move a real clunker is extra stress on your joints. Plus it’s just not as much fun, cycling is a sustainable exercise largely because it’s fun. But it’s very true that a good workout bike can be had for $100 if you look. My two workout bikes were both built in the 90s (They’re fancy but that’s besides the point).