Gas prices:

I don't really understand gas prices. Today, while driving down Harlen in Burbank, I saw a BP and a Shell right across 63rd Street from each other. The BP gas was $2.06, the Shell gas was $1.99. Gas prices are no longer academic for me; I drive several hundred miles a week, and while I don't have a gas-guzzler, it does get pretty expensive. Why do the prices differ so much, even over a recently small area? Where can I find gas for $1.85 a gallon?