@D_Rob I would disagree that DC motors would be cheaper for a hobbyist…currently. What I am saying is that we are close to a local minimum with our current designs so we will have to spend more money and time in the short term to develop it and then you will have to wage an uphill PR battle to win many over to the more expensive solutions that will eventually be cheaper.
That said, it sounds like a fun project. You just won’t want to use the standard RepRap electronics that have the stepper solutions built in. I am sure you could make a Pololu drop in that will help you out. (Of course this won’t allow for it to be backdrivable and trainable which would be awesome.) If I were you I would grab the RAMPS plans and throw away all the stepper circuits and insert your own drive and sensor circuits.
These motors cost less that $6 each with shipping. Torque information is not provided but plugged to the board @D_Rob mentioned above that can manage four motors you would have a per-channel cost of around $12 motor+electronics drive which starts to seem competitive. Still you need to couple that with the brains. Cheapest solution might be to drop one ATtiny85 for each channel to offer dir/step interface to interact directly with Marlin/Repetier software running over a Mega. I start to see that as feasible cost-wise.
I stand corrected: Stall torque: 440 g-cm (43.2 mN-m) seems to be the motor’s torque. Way way below the 4kg-cm of many steppers. Extruders would need a higher gear rate here and carriages may not achieve very high acceleration then.