I have just bought a wahoo kickr to get fit with – when you hook it up to something like Zwift or trainer road it is much easier to motivate yourself.
But that’s over your budget.
A while ago I bought a second hand spin-bike, a Reebok one that was freakin’ heavy, BUT it was boring to use and eventually I stopped using it.
It also only works the legs.
If you are after actual CV work and overall fitness, rather than just bike fitness, then look to see if you can get a decently robust Schwinn Airdyne – where the pedals and the arms link to a socking great fan where the front wheel is.
They had one at a gym I used to go to near work and I really liked it – you can vary whether you are putting most effort through your arms or legs – and the fan keep the air moving as well as you exercise.