True. But there are only a few different types of gas, and in the US alone, there are 50000 new food products introduced on supermarket shelves every year: http://www.ers.usda.gov/Briefing/FoodMarketingSystem/new_pro...
And there are also many more places that sell food than gas (proof is that places that sell gas almost always sell food too).
But with enough motivated individuals, anything is possible, price tracking included.
It is definitely doable. You don't need 100% coverage to be useful. A French company could do it (prixing.fr) although they don't currently. They allow you to scan a product barcode in your supermarket and compare it to their database to see if there is a cheaper alternative elsewhere. As baseline for their DB they scrap supermaket's online store and then complete/correct with what people scan with their mobile. Works really well in France because the market is somewhat concentrated an most of them have an online store.
Given that, they could probably track prices over time. I guess they probably do it internally and sale it on B2B basis.
