Does anybody know what is the Steam's usual cut?

No, because it's under NDA as far as I know. In fact, I believe (note: never contacted Valve, so I could be wrong) that it's negotiated depending on the game, which may explain why they don't want to disclose numbers.

Unconfirmed rumours state that the industry standard appears to be about 30% for steam, iTunes, amazon, etc.

Could be wrong, but I've read it in a few places.

Unconfirmed rumours state that the industry standard appears to be about 30% for steam, iTunes, amazon, etc. Could be wrong, but I've read it in a few places.

facebook takes 30% too from in-app purchase. so it makes sense Steam would bargain for the same, unless you can offer a tastier deal.

A recent article which may find of use.

Defender's quest - by the numbers

Whilst it does not pertain directly to the percentage question of the OP I do believe it contains sufficient material to be a worthy adjunct to this particular thread.

In addition to what has been mentioned above with reagrds Steam's cuts being variable from game to game or organisation to organisation, I have also read that Steam has also shown some flexibility in reducing their cut when a product undergoes a massive discount sale. How broadly this applies I don't know.

A recent article which may find of use.

Defender's quest - by the numbers

Whilst it does not pertain directly to the percentage question of the OP I do believe it contains sufficient material to be a worthy adjunct to this particular thread.

In addition to what has been mentioned above with reagrds Steam's cuts being variable from game to game or organisation to organisation, I have also read that Steam has also shown some flexibility in reducing their cut when a product undergoes a massive discount sale. How broadly this applies I don't know.

I bet Valve also considers taking a lower % to attract big studios to signup with them.