Keep an eye on Nashbar's stream of sales. They frequently have good rear racks from Bor Yueh or Ascent on sale for around $10 (the Bor Yueh was 9.99 just a few days ago but is now back up to $18). These both have the third support leg on each side designed to go farther back and prevent even a flexible pannier from getting into the spokes. Similarly, Nashbar periodically has had their 'waterproof' and 'ATB' panniers on sale for under $30 a pair. They're pretty basic but quite functional and have served many people well on extended tours as well as commter/shopping applications.