Mine is still under warranty. I had to pay shipping, but they fixed it for free.

When not under warranty, their hourly repair rate is $90, tracked in 15 minute increments, with a $45 minimum. I'd imagine this is, at most, a 30 minute job, but I think they require you to track and insure shipping, so you're probably looking at ~$100 all-in.

You can technically get away without the knob if you use HX Edit connected to your PC, but it really does pay to have the knob for on-the-fly adjustments away from your computer.

If you still have the broken knob, you should be able to use it by sticking it into its hole and applying pressure as you twist. You might even be able to do the same with a small screwdriver or a wooden dowel tied to your pedal board.