Theres an addon that I've been using for some time that gives you the best deal on whatever you're searching for. For example if you're on ebay looking at a fork fox it will just pop up at the top of the page and tell you "item is listed for _____, its $xx cheaper here, here and here.." It's called InvisibleHand, and is for firefox and chrome. I'm not associated with it by the way...

Firefox:
https://addons.mozilla.org/en-US/fir...invisiblehand/

Chrome:
https://chrome.google.com/webstore/d...oiemllfnlmmoko

Hope it helps

EDIT: I just had a look at the main site and it seems that it's available for all browsers:

http://www.getinvisiblehand.com/

P.S Don't expect it to work right away, once you're on a page it has to search other sites which takes a few seconds.