I find it hard to believe that money is the "only" factor. A publisher would have to be pretty silly to not consider the number of lost potential sales when considering the worth of an exclusivity deal. That won't always directly correlate to market share, games frequently sell at different ratios to the consoles they're on, but having a market share advantage (especially one as significant as Sony's right now) will certainly work in your general favor.
That said, i expect there are some exceptions, such as when Sony, MS, or Nintendo offer to help fund/develop a game that might not otherwise exist (either at all or in the desired form). Their willingness to go through with the deal is likely more relevant than market share in situations like that.
