They were kind of stuck at 25 mill for a while there, then took a jump. I think that was a couple years back. Before now, the largest amount of singing balls I'd heard of anyone having at one time was ROBIN HOOD of Baja, who had like a bunch on top of his vendor castle sky mall, all dyed red and green.
I've personally looted 5 or 6 of them from idocs, all in fel. I bought one as I gave away one ball to a friend for christmas and wanted to have the same number as I did then. I think they are more common than people think, but still fairly rare, and they represent something cool to have so they will always be in demand.