It really depends on the game. Look at past sales of the Metal Gear Solid franchise, the main ones in the series, not ones that were just "upgrades". Metal Gear Solid was a big upgrade from previous games, and it sold about 7 million on the PS1. Metal Gear Solid 2 was the next big upgrade, and also sold about 7 million on the PS2. Metal Gear Solid 4 is another big upgrade and yet it sold only 3 million on PS3, even though the video game playing audience has expanded since then. That is a massive drop in sales.
Two things can be taken from this. Either millions of people suddenly got bored of the Metal Gear Franchise, or millions still really want to play it but can't afford the $500 cost of entry (PS3 + game + tax). Now, what would happen if it was suddenly playable on a $200 console? I would say 2 million more sales easy, and I think 3 million is not out of the realm of possibility. There are many out there still really wanting to play the game, but few in their right minds will spend $500 to play it.