Your smartphone already allows you to do instant price comparisons at the store, usually by scanning a barcode. There are several apps that let you "showroom" in this way.
We don't know which, if any, will be available for Glass. But assuming one of these apps gets ported over to the new hardware, you'll be able to get price comparisons just by picking up a product and looking at the barcode.
And it doesn't end there. A couple of those apps -- including Google's own shopping app -- are able to recognize the covers of books, DVDs, CDs and games, which means you don't even need to find and focus on the barcode. So, for example, if you find a book on the shelf and want to see if it's cheaper online, you could just look at it, whisper a command, and immediately have a bunch of price comparisons floating in your field of vision. And you'd also be able to call up reviews, too, so you won't have to rely on those selectively quoted blurbs that publishers choose.