PhoneGap and Appcelerator are for building apps in HTML/CSS/JS. Xamarin lets you write apps in C# (statically compiled to native code, in the case of iOS, last I checked) that interact directly with native OS APIs.
you get a 10% discount on the total price if you buy multiple licences at the same time
You can see all here: https://store.xamarin.com/#purchase
Anyway i was not talking about the support option, but about the chance of deploy a full serious multios app. With phonegap you have that for free, with xamarin you have to pay (299*2) minus discount of 10%. Just this.