I generally avoid granting open borders due to the tourism as well as generally being overrun by a wandering AIs peaceful army. Do you find its a problem to grant open borders (betrayal, etc.) or am I just wasting 2 - 5 gpt on each civ?
If you are making a lot more culture and tourism per turn than the ai then it is worth it to pay for the open borders. If the ai makes more culture or tourism than you it is not because if you ask for more than open borders the AI will usually not pay for open borders.
Are you seeking a cultural victory?
If not, generally keep the borders closed.
If so, your tourism & culture should be so high that their tourism over you won't matter at all. In this case a one way open borders for a friendly AI wastes GPT compared to shared open borders. The one way open borders though would benefit you if they have valued your open borders being opened at 0 GPT, which often happens if they aren't friendly.