Figure out how much time the original app took to build and multiply that by the going rate for iPhone development. This will give you a ballpark figure of quot

Figure out how much time the original app took to build and multiply that by the going rate for iPhone development. This will give you a ballpark figure of quotes this client might have been given for the app. Then charge a percentage of the estimated cost that you think is fair, keeping in mind that it's a huge win for them: there is little technical and timeframe risk as 99% of the development is already done. Worst case scenario, their campaign doesn't do as well as they would have liked.