Buying is with a pro that is straight up a 1-1 exchange, but it is a business relationship and she definitely has other customers if she is selling to you.
Tricking is a more costly as you are a walking atm/doormat to a woman that isn't your spouse. Like a pro, but you have essentially a long-term lease on the box, but she could be stingy with it if she is getting you to buy things.
In both these situations, you are paying for a desired outcome.
With a genuine girl, who really feels you, none of that extra stuff is really needed and is up to you if you want to buy her stuff, not because you want sex, but you love her and want to show her appreciation.