I did this, but I haven’t used it on the DW660. I use it on the vacuum. Not sure the precise cost but yeah it was cheap.
The cheap dimmers from the orange store are rated for about 600 watts. Inductive loads are okay because SCRs switch off when the current crosses zero, so no inductive voltage spikes are generated.
I don’t think a speed controller will be able to increase torque at low speed when the tool gets loaded unless it has some external speed sensor. A brushed DC motor on rectified 110 doesn’t provide any real indication to the speed controller that it is slowing down and needs more juice. At least not through the power line, which is why some external feedback would be necessary.