If I were to now upgrade our infrastructure to 3.1 via either apt-get or via their process I wouldn't be able to upgrade to future versions without either paying for support for each blade or downloading the ISO and doing a complete re-install? And if I were to run apt-get I would get "beta" updates but not the latest stable version?
That is one possibility, but since they are not communicating about this there will be no way to know for sure what upgrade paths they will provide until 3.2 comes out. Doing a "complete reinstall" in a cluster is not that big of a deal though, unless it's one of the many releases where live migration doesn't work between the old and new version. Time will tell. If you have to make a decision today, assume the worst.
On a side-note, I'm in the US and it looks like the basic support would cost around $67 per socket, or around $134 per blade annually? This seems funny since our blades are only worth around $300 apiece but would cost $3000 apiece to upgrade.
Good news: 10 servers with 2 cpus each is 20 CPUs at 50 EUR each is 1000 EUR a year. ($1337 today. Not making that up.)
Bad news: They charge 20% VAT even to US customers. So it's 1200 EUR a year. ($1605 today.)
Probably you accidentally doubled twice.
By comparison, a VMware support and subscription package starts at $273 per socket and goes right on up to $874.
If a 10-blade chassis fully loaded is only worth $300 total it's probably time to upgrade. You must spend at least that much on electricity for it each year.
Maybe they should implement a sliding scale, 1-10 total sockets is $X/socket, 11-100 totals sockets is $Y/socket, etc.
They definitely do volume discounts, but I am not sure if 20 sockets is enough or if they will do it off the new community price. You would have to ask them ( office@proxmox.com ).