Speaking for myself as an independent provider (other ladies might be in different situations), I have to use some funds to post ads. Some weeks I don't need to post anything but when some of my friends are away on business I invite new ones to play.
Other than that, I put money back into myself, such as getting my nails done, a massage (doesn't really show to you guys but helps me relax) buying cute outfits...whatever. The biggest amount I "invest" in myself though is probably college. For me "word of mouth" between satisfied clients usually keeps the posting costs down. I think all of us providers tend to put some of our earnings back into our business, whether it's clothes, beauty related or transportation costs.