I think you could get around the latency issue by having the generated dialogue come in form of letters that you receive in-game, which would feel a lot more natural than a slow conversation. Or have some cutscenes in between the prompt and the answers. As for the price, it should probably be an optional setting, and maybe the price should be offset by a subscription or ads, as much as I hate them, but in this case it would be difficult to do otherwise, unless you plan to foot the bill of your users forever.