There are times when a site simply does not allow spidering and retrieval of their data for use on other sites. Craigslist is an example. You are not allowed to store their API data and create other searches.
Ways around this have been to just ignore their directive (and later be shut down), to use Google to search the content (and have old results), or to use multiple windows to do the multi-city searches many people want.
I think similar techniques could work to do local price comparisons or other API-limited activities. Almost no one will block a Google user agent, so you can get the data whether they specifically want you to or not.