I suspect that the marginal value of a dollar to Elon Musk is close to zero, which makes it difficult to test his sincerity in his beliefs by offering a bet.
I would structure it like this: I give him $100 right now, and if there's no AGI in 10 years, he gives me a squillion dollars, or some similarly large amount that reflects his confidence in his prediction. This way, he cannot claim that a fooming AI that renders dollars worthless will deny him the benefit of a win, because he gets to enjoy my $100 right now.
Elon is unlikely to accept this wager; would anyone like to accept it in his place?
Elon Musk submitted a comment to edge.org a day or so ago, on this article. It was later removed.
Now Elon has been making noises about AI safety lately in general, including for example mentioning Bostrom's Superintelligence on twitter. But this is the first time that I know of that he's come up with his own predictions of the timeframes involved, and I think his are rather quite soon compared to most.
We can compare this to MIRI's post in May this year, When Will AI Be Created, which illustrates that it seems reasonable to think of AI as being further away, but also that there is a lot of uncertainty on the issue.
Of course, "something seriously dangerous" might not refer to full blown superintelligent uFAI - there's plenty of space for disasters of magnitude in between the range of the 2010 flash crash and clippy turning the universe into paperclips to occur.
In any case, it's true that Musk has more "direct exposure" to those on the frontier of AGI research than your average person, and it's also true that he has an audience, so I think there is some interest to be found in his comments here.