Not expensive at all....I've actually been thinking up a system to do some of these things. There is a hardware/software package out there called Arduino...it's for prototyping/programming. It's just the type of tool to do something like this!
Right now I'm working on a box that takes in the following:
EGT - measured by thermocouple
CHT - measured by thermocouple
RPM - magnets epoxied to magneto housing, detected with hall effect sensor
Speed - magnets epoxied to wheel, detected with hall effect sensor
Throttle Position - likely a potentiometer
It may become a fly-by-wire type setup, with the throttle position measured, and the carburetor controlled by a servo. Still up in the air on that.
Ultimately, I want to incorporate timing adjustment into the system.
My primary focus is racing, so fuel calculations are not a big concern for me...but really, it's a simple case of software....with a couple buttons, you could tell the system everytime you fuel up, and how much fuel you put in....after a few tanks, the software could pretty accurately give you a 'distance to empty'.
For a pre-built solution, should you not want to design/build something yourself, check out the Veypor...I have one on my TL1000R, and it's very cool! http://www.veypor.com/