MakerBot Industries

Go get a $19 digital scale from Harbor Freight, then weigh the object or the spool.
Plastic cost is calculated around $20-23 per pound (using MBI plastic).
I use a fair amount of Ultimachine plastic which runs $9-15 per pound.
http://www.harborfreight.com/digital-scale-95364.html

Far easier in real world use than trying to calculate. It is done in both Netfabb and Replicator-G, but a scale is just so much easier and faster. It's also far more accurate.

It even has a really accurate "tare" so a box can be used or a flat plate to lay objects on.

Again, this is far easier and just a better system in general than the idea of measuring linear distance into the the extruder.

Also, take into account that even if you did get an accurate consumption value via the software or another method, how are you going to authenticate the user and then ensure the part is printed under their login? Way too complicated.

Scale, spreedsheet, and the honor system with a periodic check by weighing the spool is all you need. The scale is accurate enough that you can tell at the spool how much was used. Each person weighs the object and any scrap and records it, then check weekly the spool weight to balance the honor system. You will be able to tell if something wasn't recorded. Otherwise, I think you underestimate the complexity of either individual accounts on the printer system, integration into some acounting spreedsheet acount, and overall security and functionality of the system.

As jetguy says I also bought a digital scale, then measured several objects and I calculated a ratio about time estimated -> grams (fairy precise :) )