Most of them do it already. Many get naked for ads and other "artistic" layouts, and at the very least a large minority does out-and-out adult modeling. And that isn't for a huge amount of money either. But only so many people are willing to pay you to take your picture, and if you want to make a living, and you aren't one of the VERY few at the VERY top of the industry, then you will go where the money is, at least occasionally.
Most of the big names have come forward and said that to get ahead, there is some shady behavior. There've been models who say some of the biggest names in the industry, like photographers or those working for designers, will basically tell the model "sleep with me, you'll get this ad." And many of them have had to do that at one point or another
A lot I bet...including me! LOL. I mean what price are you talking about. You can take a picture of me and put it up on Times square for one million dollars. How does that sound? If Kramer can do it, why not me?