These figures are from actual weigh ins off of a digital scale from new stock from an authorised Mavic and Shimano dealers (Chain Reaction and Duke's Cycle both in Toronto, Canada):
RS 21 rear rim weight alone: 506g ! Heavy! But strong, solid and asymmetrical w a generous brake track.
Dura Ace 9000 C24 (WH-9000?) f/r: 620/840g
Ultegra 6700 Alloy f/r: 740/971g
RS80/C50 f/r: 835/1060g
Older Mavic Aksium Race Rear Rim, silver 20h eyeleted and heavily USED/worn: 493g