The "G12" specification is not for oil.... it is for ANTIFREEZE.
I assume since you are asking, you realize that you should never EVER use anything else in your VW. (intermixing different antifreeze can cause 'gelling' inside the engine and overheating.)
I use pentosin brand "G12" in all my VWs. Here is the link to their website.
If you cannot find it locally, I HIGHLY recommend IDparts. Here is their website:
To speciffically answer your question: The G12 antifreeze is about $12.95 for 1.5L bottle.... This MUST be mixed 50/50 with distilled water so you end up with 3L of antifreeze.
Source: personal experience