It depends if the app is written as native (Objective C for IOS and Java for Android, 2 very different source codes and languages/API), or, if the guy is going to use tools.
For example, just knowing web (css, jscript) you can use Adobe Phonegap to create ios/android apps:
http://phonegap.com/
A very nice article:
http://mobile.smashingmagazine.com/2...art3-phonegap/
Top apps you should make native (games etc.), because phonegap is not so fast or supporting all, but native apps should cost $thousands depending what you do. For most not-so-top-of-the-edge apps, you can use phonegap and should cost like a web app.