+1 this idea. In ReplicatorG/Skeinforge you can set the printer's operating cost/hour and the filament's cost/KG, and it'll estimate print time and volume of plastic, and print out a cost estimate for each sliced file. This can be handy if it's the printer is shared, or if you want to keep a sense of the cost of prints. And it's quite convenient that it writes it to a text file next to the gcode, so you automatically generate them all the time, and look at them when needed.
Of course, actual print time depends on the printer's configuration, but even an estimate is useful.