Well if your hurting for money that bad that $10 could break you then I suggest that you end your subscription and bank that extra $5. There are far more important things to spend money on than a game. Priorities.
I CAN pay the subscription right now. I could go on, subscribe, and nail a couple of T34/76's within 10mins, but I CHOOSE not to, because I should be saving it now.
But now that you have brought it out into the open lets ask what your really asking for. A cheaper way to pay so all of your buddies in intermediate school can play too ( you mentioned not being old enough to work by law). Again, I believe if you can't afford it, you shouldn't be spending it. It's a good lesson to learn now so later when you start thinking about buying a car you'll look at those that you can afford.
Can already buy a car, and do they really have 15yr olds in intermediate school where you live?
The same goes for the game. Its entertainment, and nothing more. My kids use to earn their game subscriptions by doing chores around the house. Of course they didn't realize that once they made the deal I had them by the ...ummm ... well you know whats

"Do your chores or I make a call to Blizzard right now!" LOL!!! taught them a little responsibility.
I realize it's entertainment, and it doesn't take anywhere near top priority. Lol, had something like that when I was 10 with the PS2. Mom would always threaten to take away sly 2 band of theives. hehehe 5 years ago I was playing sly, and now I'm blowing up tanks....
Instead of looking for ways for some one else to make things cheaper, why not looking for ways to "earn" your way to pay the bill. I know, it's easier to have someone else fix it than it is for you to work at it yourself.
While I would get benefit from it, I'm just over the edge of not playing. I can make money almost as fast as I spend it on this game so I could get away with it for about a year before it caught up with me, and allmost all previous work will have been for a video game instead of a car.
Too easily abused. It should start at $15 a month. Paying by the week/day/hour could be an option but not a benefit. Paying up front and in full is a reward in a cheaper price.
As far as pissing off customers goes, if you are not currently subscribed you are not a customer. You are a potential customer or a former customer and, in your case, an occasional, former, potential customer.
wrongway
How would it get abused? You can pay up to 15$/month at which point you make no more payments for the rest of your 30 days. If you only play for 2 weeks, you've saved half of next months game funds by not playing that extra week.
And it swiches about every 3-4 months seems like. And i think I've had my Nemisis account the longest out of all of them (a whopping 11 months)