It depends on your budget which will somewhat narrow your choices potentially. From there it depends on your case and what size fans you can fit there, is it 120mm only, is 140mm an option etc. Cfm (cubic feet of air per minute) is a measurement usually taken in free/open air tests meaning no restrictions like fan grills or filters.
Noise is a consideration, some fans move similar air at lower db than others. Some have the same db (sound pressure level) but in different tones, high pitched noises can seem louder than lower pitched tones so seem quieter. A whistle at 30dba is going to be more intrusive usually than bass at 30dba.
Some fans have better more reliable bearings than others, cheap fans tend to use cheap bearings that either causes the fan to slow down or wobble, wears out, can create grinding noises in a relatively short period of time compared to nicer bearings.
High cfm fans with low static pressure (think 'muscle' when moving air) can bog down in a hurry vs a fan with higher static pressure. A rear exhaust fan is one of the least restrictive fan positions on a pc case but cases all have fan grills on the back to prevent sticking fingers in the blades. A fan rated at 60cfm may only blow around 50-55cfm out the back of a pc case because there's 'some' restriction there.
If all you're looking for is a cheap exhaust fan, single speed, maybe something like one of these.
http://pcpartpicker.com/product/sdZQzy/cooler-master-case-fan-r4l2r20acgp
If you want one that's pwm (variable speed 4pin) maybe one of these.
http://pcpartpicker.com/product/DPZQzy/cooler-master-case-fan-r4bmbs20pkr0
http://pcpartpicker.com/product/KzQypg/arctic-cooling-case-fan-afaco120pcgba01
All of those are around $10 or less.