Even Mosley knew it wasn't a knockdown. I can only guess that the ref thought the punch Mosley landed like 3 seconds prior caused Pacquiao to fall down in some delayed reaction.
Reports where that it cost around $60. I watched a live feed from p2p4u.
Mayweather has been and can hit. Mosley whacked the mess out of him twice in their 2nd round, but he recovered and went on to dominate Mosley. After watching last night, I am convinced that Mayweather would still defeat Pacquiao by decision. However, there is no doubt that Pacquiao can put Mayweather to sleep if he lands flush. I've seen Mosley take punches that looked a lot worse than the one he got from Pacquiao that sent him hurling to the canvas, but he claims that's the hardest he has ever been hit.