Hm, I guess it depends on how long the overall story is...?
There's this one otome game for IOS called "Ephemeral." I don't exactly remember how much each story costed, but it was very reasonable for such a good game. (I think all the main stories put together was about $15 or so...? And that's for 4 "romancable" characters.)
That being said, my absolute max would be $20, give or take. (Sorry, I would be able to pay more, but I'm kind of too young to have money of my own, so...ahaha...)