For me I've just recently started offering it. I post out used underwear they can buy from me for ?20 plus postage I'll wear it all day and masturbate then package it up after. For an extra ?10 I'll include a polaroid taken while touching myself in the underwear (no face shots) and I make a lipstick mark on it. I just buy the bulk packets from Asda they don't look terrible but they aren't anything fancy.
I charge ?30 extra per booking if they want to keep my underwear after and ?40 if they want to request underwear type etc. I make more allowances for them doing things with my underwear in this case such as tearing it and leaving it on for the booking,panty stuffing etc I also use better quality underwear for this thongs etc especially with requests. What you charge just depends on how they want it if they just want to have used underwear delivered then you can get away with using the cheap bulk buy store brands.