I would gladly pay around $10 a year, maybe even $15. My Pebble has been worth it. However, I am now going to return it because if the next Android update breaks the app, it will become a paper weight.
I mostly just use it with cycling (MapMyRide supports it) and for the music app. I realize their servers/app store will shut down eventually (I hope they at least decide to take whatever money they have left and keep the servers running as long as possible after paying off people/debts) and at that point it will be difficult, if not impossible, to get it working with new phones.
I'm going to do what I can to keep mine working until then though. I prefer the simplicity of a Pebble to any of the Google/Apple offerings.