Unfortunately the documentation doesn't mention how it's implemented (at least I couldn't find anything), but considering the steep pricing, you're probably right with your assumption.

Hopefully they'll be able to switch to Server Name Indication (SNI) in the near future as that would save a lot of IP addresses (and, if that's their biggest cost factor, allow them to lower the price). I think Windows XP is the biggest obstacle w.r.t. SNI, but thankfully XP will be EOL'd soon(ish).