I would say : Ask them, look up their wishlist ! Maybe its written on their website ?
If it has to be a surprise.. maybe CASH hahahahahaahah, offer to pay for her advertisement fees ? A new boudoir photoshoot for her social medias ?
Does a SD see his SB often ? If its once a month, 10 is cool i think ! even twice a month would be reasonable in my opinion. It gets complicated if they want weekly/daily meentings hahah