Cooper Allan and Horze sell great quality, cheap breeches.
HorZe ones.. B Vertigo by horZe
I found Ashley brand breeches on sale for $12 or so dollars on Horseloverz during their 4th of July sale. They have a lot of sales on the Horseloverz website.
I also love riding sport (and when they wear out you can exchange them for another pair for free)