Ask HN: Costing cloud backend for mobile apps?

I'm about to release a cross-platform app (iPhone/iPad/Android) and one of the main features is cross-device sync. I'm building this up with Parse at the moment and the free tier is pretty generous, but... I worry slightly that after a while it's going to start costing me serious money.

The app will be paid but only around 0.99 - and that's a one time payment (so, 0.66 to me after app store cut). If I get a lot of heavy use then a distant 0.66/user isn't going to be much help.

Just to be clear, the app alone is the product - it can't make a loss to help promote some other entity.

As far as I can see my options run to:

1. Increase the initial price (still chance of this not covering later operational costs)

2. Charge annual subscription for sync (would anyone pay for this?!)

3. Worry about this later - I'd need pretty good traction I know, but my earnings are pitiful and come the day it may happen, it'd wipe me out.

What have other devs done to mitigate this?

Yeah, those are possibilities. I don't really want ads in there if I can avoid it though (especially with it being a paid app) - and a future payoff is exceedingly unlikely. It's the kind of app that could grow to the point where it costs me, but not where it would blow up to even a milli-Instagram.

I think some kind of premium option, whatever that may be, would be the best bet.