I'm currently looking into it because I desperately want them as well. Anyway, here are some preliminary cost estimates, I don't have the part numbers:
Airbag with cutouts for audio controls: $505.52
Left hand audio controls: $67.39
Right hand blank control: $14.83 (not sure about this one)
Leather steering wheel (optional): $100 (if you can find a nice used one on eBay) to $600 if you want to order a new one.
I bought a leather/wood steering wheel recently, so I've just been waiting to find out about the wiring before I go ahead an order the parts to add the both the audio controls and my new steering wheel in.
EDIT: I checked my stereo and it looks like it's already prewired for the steering wheel audio controls. The R5 connector is in place and has the 3 wires corresponding to the audio controls. I guess Toyota thinks it's more cost effective to include the wiring on all 4Runners since the majority of them are equipped with the audio controls.