I think that Google has had to make significant infrastructure changes in order to support SSL on GAE. This has been a long standing feature request so I for one am happy that it is finally here.
SSL was always supported on *.appspot.com URLs. Just not on your own domain name. The reason is not the 'computational' cost but the fact that for SSL to work the server that handles the requests for your app will need that SSL certificate.
So how does that scale when you've got a million of those machines? Not to mention the fact that you need to distribute the certificate everywhere. Then there is the problem that its not just your SSL certificate but one certificate per app that wants SSL and the fact that they will need to allow app owners to purchase, upload and replace the certificate.
The SNI/TLS option is cheaper but requires OS's and Browsers etc. to support it because this solution uses an extension called Server Name Indication (SNI) that allows Google to 'select' the SSL certificate dynamically based on that 'server name'. Older implementations of SSL will probably not support this.
The VIP option is more expensive because it assigns a dedicated IP address to your application. This solution does not require SNI.
So I guess that if you want to be absolutely sure SSL will work everywhere, then go for VIP.
The cost of any feature is just another thing to consider in your business plan. I am not convinced that the amount you are referring to is going to be a show stopper for those that would really benefit from the feature.