Is it legal for me to pay a woman for a photo of her boobs in my face for a facebook fan page?

Facebook does not allow such photos to be posted, so save yourself the money.

If the woman is a competent adult who understands what you are asking her to do and how you will use the image and she provides written permission for you to photograph the two of you together and publish the photo on Facebook and the site permits the posting of this type of image, it is probably legal to do so. Whether you make money or not is irrelevant. Two caveats: first, children must be prevented from seeing the image and second, pictures like this can reduce your ability to get a job, stay employed, obtain insurance, participate in a number of clubs and social activities, date and find a significant other, etc. Once you post this image, it can't be taken down and you have no control over who sees or uses it. You need to decide if a lifetime of limited opportunities is worth a moment of being seen in this manner.