You might want to check the sale selection at places like ebags.com. I think it'll be slim pickins at $50. 17" bags are less common, so only the better manufacturers tend to make bags for them.

Brenthaven is one of my favorites, but it's around $150, I think. For an informal bag, Tombuk2 is good. Other favorites include Tom Bihn, Waterfield Designs, Marware, InCase, STM, Briggs and Riley, etc.

Many like to get bags from Costco and seem to find them at a good price/value there.

A good site for reviews of several laptop bags is bagreview.com.

happy hunting!