Edit: Looks like some guy already did this: http://blog.makezine.com/2011/09/26/toilet-paper-printer/
wifi chip plus antenna - $30
microcontroller + associated bits - $8-15 or so
stepper motor drivers (you need two) - $8 or so
stepper motors - $6 min
printhead and ink - $12, replaceable
Battery and charging circuitry
And that's not counting manufacturing and profit. Would you pay $70+ for printed toilet paper? If a couple people would, I'm willing to design the device, sell a couple, and open source it.
If you are actually interested in doing this, I'd be up for doing the project. I actually have the motors, motor drivers, microcontroller, and inkjet cartridge full of red ink already. I also have a 3d printer for mechanical prototyping, and a good supply of toilet paper. If you are interested in working on this with me, email me at kliment.yanev at gmail.