That's certainly an interesting idea. The hard part is knowing how current (and accurate) their data is.
I played with this idea once. Our MLS here usually includes the most recent property tax bill. Dividing a listing's tax bill by the town's normalized tax rate will give you the tax value. If you then divide the asking price by the tax value, you'll get another percentage.
As you do this for a particular town, you'll see that some houses are listed at close to their tax value. Some are listed higher, and some lower. If you sort by this percentage, the ones with the lowest percentage are the "best" deals in that town.
It would be really great if we could figure out a way to get this report directly from the MLS. Just another service that a Realtor could provide.