Strange that the section about Visual Studio Professional says "contact us for a quote". Can't you just purchase that from the Microsoft online store?
edit: typo
You can buy both it and Enterprise online. Not sure why Xamarin says to contact them for a quote, except that Xamarin's sales people really like to talk to people.
So if you're a VS user already, it's likely bundled with an MSDN subscription. Just link it to your Xamarin account and you're done.