I agree with staying away from gel saddles and gel covers. Gel seems so promising in the beginning, but ends up being painful in the end... literally.

The $50 price tag is going to be pretty limiting for new saddles, unless you can find something in good shape on ebay, but even then, the $50.00 is going to be gobbled up in freight.

For a commuting saddle, I suggest the following:

Choose a smooth surface over a plush surface. You want to be able to slide back and forth on the saddle
Choose a springer saddle Your back will appreciate the shock-absorbing qualities.
Choose a saddle with a solid firm cover rather than a soft foam or gel.
A wider back with a shorter nose is good for commuting as opposed to long nosed saddles or other strange geometry.
Leather is an ideal saddle cover material if you can afford it.

My favorite saddles for commuting are probably Lepper saddles