Google maps can now factor in cost of driving with directions.

This provides a unique opportunity to make your product useful. Above and beyond simply providing maps to stores for cheapest prices on every item, you should take into account the cost of driving to these locations before recommending users visit them.

For example:
I'm looking for milk and cereal.
Cereal is 1.99 at market 1 and 1.50 at market 2.
However, milk is on sale (savings of 1 dollar) at market 1.
BUT, the cost of the trip from store 1 to store 2 is 1.15. Therefore, it is NOT a sale which makes it worth a trip to two markets. Taking advantage of both savings would be the most expensive option when accounting for the cost of the trip. Therefore, the map should take the user only to the store which saves them the most money (i.e., the one with milk on sale only).

This is a feature which would help make your product a killer app. Email me if you'd like to talk about it more:
Derick Valadao
dvaladao@uwaterloo.ca