PetDMC - Rheostats in the 10W range are not cheap either, usually in the $15 to $20 range. If you aren't going to be constantly adjusting them, you would be better off either just running them at 7V or using adjustable power resistors ($7 range) from a cost standpoint. Examples of choices are
here.
Regarding getting fans to start at lower voltages, you can solve that problem by putting a 5 to 10 microfarad capacitor across the resistor to supply them with the startup juice they need
