Lot many ways are available to make money but i look back into your no's i don't feel that asking directly user to pay for service will be not a good idea. So you can create a new paid feature on top of this as i feel you are having enoguh time in terms of getting more no of user. So what i feel for current no you can go for below option :
Once you are having enough no of user you can launch a paid version as well so at that point you will not have a fear to loss your user because you are creating a feature on top of your current feature list, so user will be happy anyways and they are having choice.
This article will also help you at some point.
Regards : Md Zaid Imam