If I'm not mistaken, MSDN subscribers (not BizSpark though) also get a discount on Xamarin licenses (or maybe it was just their university learning subscription)
Apple tried blocking certain platforms in the past, at that time Xamarin wasn't included since they are following the guidelines set in Apple's developer terms of use. Apple changed their mind very quickly about that decision too.
Xamarin still requires a mac. Your code is still compiled using LLVM (or whatever engine Apple will introduce later). The resulting app that is uploaded to Apple looks just like an app that was built using XCode.
It will be interesting to compare a simple app and see the difference in the resulting package between Xamarin and XCode
I think the only important difference between the $300 and $1000 licences is the Visual Studio add-in and while I prefer VS to Xamarin Studio, I would assume the starving devs at a start-up will be fine with it.
The only way your math makes sense to me is if a start-up only plans on releasing for one platform at first. If you're going to build an iOS app first and only move on to Android if that iOS app is successful than it probably doesn't make sense to use Xamarin.
If you're planning on doing both iOS and Android at the same time (and maybe Windows Phone as well), the added developer hours (same dev or an additional dev) to build the same project in another language instead of sharing 50% of the code and just building another UI would probably cost you more.
I never heard anyone for Xamarin selling it as a game dev solution. IIRC Unity is actually running on the same Mono engine from Xamarin (it's an open source project, but I think they're running it). Last time I checked some of those other solutions had expensive licenses too.
Xamarin is still too new to see lots of job postings looking for that skill. I also think that in the near future companies using Xamarin will use it for LOB apps. That's a big market for .Net developers building desktop software or internal web sites. A transition to Xamarin might makes sense for both these companies and these developers.
The bottom line, products are targeted at specific customers. Xamarin could have decided to go after the start-ups, but since most start-ups are content with releasing an app on a single platform first, the proposition of getting additional platforms with less work isn't that enticing. It's much easier to sell the product to an enterprise as their cost for multiple teams with different skills would be a lot higher than the Xamarin license.