Android : How to earn money from application by remove ads?

Hello in my free application i have AdMob banners.so,Many users ask me for remove the annoying ads.i want to give them the option to remove the ads but only by paying money. my application is very useful for the users in my country and i work very hard to give them that. how can i do this? where should i need to start? i need to publish a new version of the application? and remove the ads are reliable? I tried to search some information on the web ,but i even dont know where to start. thanks for the helpers.