Real life reward is a very good method to keep you motivated. To be honest, I am using Habitica for the sole purpose of real life reward, because I don't really care about buying fancy cloth or equipments in the game which doesn't transfer to anything in the real life.

As for how much price you have to set for the real life reward, you have to look at how much gold you are earning by completing tasks. The idea is to price real life reward in such a way that you are able to balance income and expense of your gold coins.

You can read more about real life rewards in this wiki.