In this case, if they kept the same setup, I can see that it would be prohibitively expensive to pay for nginx plus on every single one of those VMs. I'm not sure of the nginx's license terms but if they count VMs as a separate server - probably a no go.
A better solution might be to turn all those VMs into application servers and put a big meaty nginx reverse proxy in front of them; that sounds like it would reduce the nginx bill but could be an awful lot of work. Then again, I presume they have automation, you'd hope so with hundreds of VMs...
My point is that it's likely not quite as simple as a grand a year, which would likely be a no-brainer if it was a site license.