The number of subscribers (loyal readers) is not the only indicator you should take into account. Other important factors are the total number of blog visitors, the attractiveness of the reward to your audience, and the cost.
For example, you could start your contest off with our free trial or the $19 plan and offer a $20 amazon gift certificate as a reward to the participant with the most new signups. Then, at the end of the month (or any other period you choose) you can reward the winner and analyze whether the results were worth your investment. Keep in mind that there in no set time the reward needs to be given out, so if you want to give it out every 90 days or weekly-just pick what works for you during setup.