As has been said many times on here before, WG's are not working to "meet new people" , "insatiable desire for sex" etc, we are in this for the sole purpose of making money.
A good/savvy WG will not try for unadvertised extras because she will be hoping for return custom. I can't imagine anyone returning to me if I tried to charge him extra mid-blow job! It would put him right off. If he tried for a service I don't offer in work (for example CIM) I'd simply say I don't offer it. If the WG is only planning on doing this for a short time or is soon retiring from the work, she could get away with extracting as much money as she possibly could from each customer because she won't be interested in repeat custom.
Any business-person with half a brain will treat the customer well for the reason that they want repeat custom (meaning extra £££££s). Any WG planning on doing this more than a few months with half a brain will do the same, treat the customer well, although strictly maintaining her own boundaries so she doesn't end up feeling burnt out and resentful and making sure she is not falsely advertising stuff she doesn't offer.