Is there an iPhone app that lets me scan barcodes to keep track of how much I'm spending?

It just seems easier for (lazy ole) me than a calculator. If possible, I'd like one that can figure in tax too.

Just off the top of my head I'd say there may be one, but it would most likely base it off the retail price, which is rarely what stores actually sell products for.

You can likely find a lot of barcode reading apps, but the databases that link the barcode data to actual prices are usually the property of each store. The same barcode might have different prices at different stores. I doubt that there's a database large enough to contain each and every barcode you might come across, but it isn't beyond possibility.