During the season they get injured, they pout, they goof off. But, when the money and their reputations are on the line, they tend to come through. Not always, but enough to make a difference. There are definitely some lazy players with big, fat contracts who do little to prove their worth until it is questioned. But that is only half the story.
Money buys better players, sure, but money also buys old better players.
As a general rule, older players tend to get paid more than their younger counterparts, thus attributing to the payroll of playoff-winning teams. Additionally, there is some truth to the idea that the playoffs are essentially one big coin-flip, and veteran leadership just might be enough to tip the scales. Anyway, getting to the point, the reason the teams with higher payrolls have such great playoff success is because they can go out and buy veteran talent. Well, the relationship would almost certainly be directly proportional. If the market value of a team quantifies the total amount of money the players on the team would collect on an open market, and the amount of money they would collect on the open market is directly proportional to the talent they have, it stands to reason that the team with the most total talent would have the highest total market value.