I guess one way would be to require the user to type in the price of the object they're reviewing, to seed the database... although that would only work well for a community with a lot of Android cell phones.

Now that I think about it, a database telling which products local people are comparing would be pretty valuable to local stores... They might be willing to trade pricing data for search data? Or a "featured" listing position?

GoCart gets the local prices from the website itself. Several merchants' websites have a "check local inventory" feature where you supply the zip code to get a list of stores. Check out the video at http://www.biggu.com