I think we are either talking about 2009 or 2010 here. Year over year the Ps3 could be down by like 1 million units in September alone but I don't know if it'll be up year over year or not so I'm not going to speculate 
But if we don't look at peak years but at peak time instead I guess that was after the release of the Slim model in late 2009 to early 2010. I know Sony can still cut the price next year but I doubt they'll drop it to 199$. The PS2 and PSP profit will be down next year because the platforms will sell less hardware and software. And Sony really needs to focus on profit so they can't drop the price by 100$ if they are just making 50$ of profit per unit. Additionally if they release a "Slim slim" version of the PS3 that is likely to have less of an impact compared to the first one (just like with the Gameboy Micro, the DSi and the PSP 3000).
A price drop next year is going to boost sales but it's not like the console will stay at the current sales level forever. There are no "fix sales levels" for each price point Naturally Sony will have to drop the price next year to maintain the current sales level not to increase it. Additionally with the 3DS on the horizon and the next gen not that far away anymore (no matter when the next generation starts 2011 will always be closer to it than 2009 ) I doubt they will be able to repeat the excitement over the Slim in late 2009. That doesn't mean it has to drop off drop off dramatically in 2011, it could just be down by half a million units or so but it would still be down
So I'm gonna say 2009 or 2010 but I'm not going to to go for one. I expect it to be within 10% of 2009 sales, though 
