
$15 a month is my standard/maximum I'm willing to pay. But it also depends on the subscription model. If the price is simply determined by how many months at a time you're willing to pay, I'm okay with it. I've played both Korean and even Western MMO's that had "bronze premium, silver premium, gold premium, platinum premium" and similar 'services' that provide HUGE bonuses over those who don't have these premiums. Those are incredibly difficult to stomach.
So in short, if it has a single subscription plan I may be interested in it depending on the game, up to $15 a month, as long as the bonuses aren't so ridiculous I feel kinda sleezy for using it.