Breakeven point between Standard and Datacentre
I calculated DataCentre edition as 5.45 times the price of Standard ($4800 versus $882) which means DataCentre is cheaper when you have more than 10 VMs on a two Processor box. If you have four processors then you need to buy your WS2012 licences (Standard or DataCentre) in pairs and you would find DataCentre edition cheaper with more than 20 VMs on a single box.
Also, if you have Windows Server unser Software Assurance and you have more than two processors per box, do an audit and tell Microsoft then they'll give you the number of WS2012 Standard or DataCentre licences you need to cover your current hardware. (Details in the FAQ.) If you don't say anything they will assume 2 processors and only give you one licence for WS 2012. If you're thinking of upgrading your hardware to more processors, do it soon, before Windows Server 2012 ships, so you get the extra licences as part of your SA agreement or it will cost you more to buy the extra licences later.