Although not perfect, Froogle is a good way to find out the prices of something before you purchase it. It's similar to My Simon, which searches hundreds of online stores to find the best prices on the item you're searching.
Froogle has also launched a mobile version of their site, so that if you find something you like in a store, you can compare it's price to those listed on Froogle.
While these sites are great at showing you how much items are priced, they don't show you how many people actually purchased the items at a specific price. Maybe the lowest priced item isn't a bargain because you have to wait four months to receive it.
A site that allows you to see what your neighbours spent on the things you're looking for would be truly powerful.
