Dear Sir/Madam, your project sounds interesting and simple enough to implement, I would be curious to see the UI before making a real estimate of the work effort, and also what features the free and paid versions offer. I have developed an application on Google Play called Looping (see my profile) which had free and pro versions implemented however this feature was removed later, so I do have experience in that field. I would be happy to hear back from you with more details and the UI screen designs.