There are two ways to approach this. You can try to get a handle on how much they are willing to pay, charge them that and hope that the number of hours that it takes results in more than minimum wage; or, you can use the more traditional scientific approach.
Either way, you need to get their agreement to the requirement before you can come up with a cost. I suggest that you write down your current understanding of what they want (and/or what you are proposing) and have them review it. This could go back and forth a few times before you get there. If you are going to be scientific, once you know what has to be done, do an estimate of the time to do design, development, testing plus all of the customer interface time, then add a healthy fudge factor for the unknowns (and potential re-work once they see it and decide they need changes). Once you have the number of hours, then you need to come up with an hourly rate. Someone may suggest a good rate for an Android developer. If not, then you need to come up with a number that is somewhere between minimum wage and $100 / hr. Your experience, your location and what the market will bear all come into it. Your hourly rate will probably be in the $25 - $60 / hr range.
