Binding of Isaac was so incredibly boring on my computer already, that I can't see myself paying a whopping $15 for the mobile version. If it had been $8 I would have probably jumped on.
It's definitely not for everyone but I'm glad they went for that price. It wouldn't be shocking on other platforms and here it's the real deal with every feature of the PC version (up to Rebirth) being included so it's only fair that it's launching at that price.