That's true, but in many cases, it's not the specific $ most think it is. Outside of the top few players in each country, more often than not, it's the $ they could potentially win in prize money that is motivating them. However, their frequent choice of Head and Wilson is not purely an indicator of racquet quality (especially at the stock level), it's an indicator of the fact that those two companies provide a greater degree of customization for pros than most of the other manufacturers.
While 28 of the top 100 use Head, and almost as many use Wilson, a lot of them are not paid cash to do so. However, both companies offer their ATP players "pro room" services, which is a large reason these two companies are so popular. To a top pro, having his equipment be reliable, predictable and consistent is worth lots of money, and they take these services in lieu of payment in a surprising majority of cases.
The top endorsers (e.g. Djokovic, Federer, Nadal, Roddick) can make a lot of money by using a particular racquet, but the amount to be made drops off precipitously after the top few guys in each country. For the majority of the top 100, the most important thing is not compromising their results, as that is where they make their money.