It could be also that Xamarin didn't want to use pricing it has no control over, e.g. what Microsoft dictates for commercial versions of Visual Studio. I assume that "contact us for a quote" would take you to the MS online store through their sales personnel anyway.

edit: typo

It's likely the case that if you need a license of VS Pro you probably need multiple (for multiple developers) and then a volume discount may apply.

I think VS Professional has been discontinued. The contact us for a quote is probably to cover legacy users.

VS Professional is not discontinued: https://www.visualstudio.com/products/how-to-buy-vs

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.

It's free in your MSDN if you're subscribed. I downloaded them all :-)

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.