I guess something like 5€ + shipping ... haven't thought on any detail yet, but that should fit in a regular bubble envelope.

I was telling myself it would also be possible to make it cycle automaticaly between each position : all lights off / only projectors / all lights on, with any timing required on each step. That would make R2 more "alive"

Further mods could also be to set up independent control for the projectors : I always wondered why the middle step was "projectors on + standard lights off", and not the opposite.
One could also want to use the remote control to control the lights instead of the sound, unfortunately my R2's sound system never worked from the beginning, and I'm having a hard time to figure out what's happening with the remote control...
That would require more substantial hardware modifications though.