Have you tested your pricing in your market? This post explains how to do it:
http://www.signs101.com/forums/showthread.php?62708-What-formula-do-you-use-to-quote-a-job&p=673880#post673880
So, for example, quote the next 20 banners at $5 a foot, then up it to $6, then $7, then $8, then $9, and...