Not sure I'd try to extrapolate from what Elon had to say in the interview because its hard to tell which number he is basing his estimate off. It could be 30% of the €40k battery price the interviewer gave, or 30% of €72k base price of the car (given by the interviewer). Or Elon might be thinking in terms of dollars meaning 30% of $72k.
It gets worse when you consider that there are two battery packs, but one of them accounts for almost 80% of sales (the 85kWh) which means when you think of the pack cost for the Model S you generally think in terms of the 85kWh pack, but the price the interviewer gave was for the 60kWh model, which might skew a quick attempt at mental math in the context of an interview.
But the most important thing to realize is that Elon is very sneaky and secretive, despite being so seemingly open about many things. He has a built in incentive to portray the battery as being cheaper than the public realizes, but also to portray it as being more expensive than it actually is. The ambiguity in the pack price is deliberate, and has been being cultivated for years by Tesla. When you factor those incentives in, I'd expect any statement by Elon to be both ambiguous, and to point to a higher number than reality.
That is even more true when you consider that Straubel has previously stated that the price of the pack is "much less" than 25% of the price of the car "in most cases", and he was unambiguously thinking in terms of the U.S. price of the car (as opposed to €72k Euros, or the €40k pack cost given by this interviewer) . That underlying cost doesn't change just because the car was put on a boat and floated over to Europe and given a higher price in Euros. So the 30% number (let alone 35%) is almost certainly wildly high if you are trying to calculate the pack cost in Euros.
