Some answers (And dont hold me to this :-) ) :
1. It is free with all ESXi Versions except the FREE ESXi Version.
2. It is activated with you ESXi Version.
For the vC Ops Std and above there is a Per VM price and then there is the vC Ops Suite that is per socket lic.
Some upgrade options here :
vCenter Operations Manager Foundation is now included with every vSphere edition. It is provided with your vSphere 5.1 download from http://www.vmware.com/go/download-vsphere. VC Ops Foundation is backwards compatible so it'll work with vCenter Server 4.0 U2 or higher.
To make things simple, VC Ops Foundation does not require a license key. And because it is the same virtual appliance that ships with VC Ops Standard, Advanced or Enterprise, you can simply upgrade by entering a new license key if you decide to purchase one of the higher end editions.
Hope this helps.
Great, thanks for the clarification.