I would say the add on sales would be close to Wii Sports Resort sales times about 2 or so (about 64 million Motion Plus add ons sold considering I believe every copy of Wii Sports Resort came with add on until perhaps later in the sales life then perhaps it came with a Wiimote+) until they finally released the Wiimote+.
At least they put the motion plus into the reg Wiimote design. I was never that much of a fan with the add on but it didn't really hinder any of the games in Wii Sports Resort. I pretty much have to take it off for any other game (still have a few old reg Wiimotes). I still have a few Wii condoms XL size for the Wiimote and add on. Screw the Wii condoms though because I like the feel without them.