I want to build a gauge for my car that is sort of a taximeter.

I want to be able to punch in the price I just paid for X gallons in the tank, and then have a meter tell me what it's costing to drive from A to B.

Does OBD provide the kind of data needed? (Miles Per Gallon/Miles Driven)?

I'm sure the data is in there. Google abounds with things like this:
http://www.obdscan.net/obdscan.html

I know the OBDII can provide the miles driven, it's displayed right there on the odometer. But I think it would be easier to just buy one pre-made calculator

http://www.scangauge.com/

The only problem with an OBDII based unit is the inaccuracy of the measurement. Just changing the tires can throw it off. A GPS based unit would probably be more accurate.

That's the one I had in mind, DTG!

"Cost per mile" right here:
http://www.scangauge.com/support/xgauge.shtml

While it could be fashioned, OBD II is very sensitive to the voltage drop across a given circuit, so unless you had a detailed wiring diagram for the vehicle in question, at the very least you'd very likely generate an error code from the branch that you tapped into.

Jim

The ODBII interface chips take that into account. Usually when building a gauge you buy a interface chip and it converts to rs232 or something more computer friendly.