Try CraigsList or something similar. Lots of people get rid of trainers on the cheap there - and you can even advertise that you want to buy one.

Turbo (wind) trainers are usually the cheapest and noisiest. A cheap one may offer up less resistance than a nicer mag or fluid, but these can be obtained around the $100 mark sometimes. You may not be able to hear your spinning DVD over it though....

Magnetic - less noisy than wind a good magnetic trainer should give you decent resistance, though if you are very very powerful you may want to go with a fluid.

Fluid - least noisy and the feel can be a little better than the other two - a little less abrupt. These are generally the most expensive of the three and the cheap ones can develop leaks!