German application developer FutureTap has bought Where To, an application that lets users find points of interest around them using their iPhone’s GPS, from fellow iPhone developer Tap Tap Tap for $70,000. As Venturebeat reports, it’s one of the first instances of an iPhone app being sold.

What’s even more interesting is FutureTap’s founder Ortwin Gentz gives a very thorough run down at his blog of why he decided to buy Where To rather than to build his own app, and the methodology he used to value it and make an offer for it. As he notes, “This is tough. There is not really any experience how to value an iPhone app. If you offer too much, you’ll never get your return on investment (ROI). If you offer too little, someone else gets the deal.”

According to Tap Tap Tap, Where To grossed “around $200,000 over three short months in the App Store.” Gentz eventually settled on using EBIT multiples and figuring out a virtual “product EBIT” for the app. He estimated yearly EBIT of $45,000-50,000, which he says will hopefully mean his $70,000 offer will pay off in 18 months.