It is a big deal, not a huge one but a big deal indeed.
I just looked it up and according to Wikipedia he has a net worth of $500 million, so he can afford to lose that $3 million, although it is 6% of his total net worth and it must have hurt. Imagine your net worth is $100,000 and going in drunk, you thought you lost $160 when you actually lost $6,000, something like that.
Let's not talk about the numbers here, it's only a big deal if he feels it's a big deal for him. And also, we don't know what is history in gambling, what if Dana despite being reach have won a lot of money in gambling prior to that lost, so he won't feel the pain or anything.. We easily react and we know part of the story yet, but if we compared that to our lives, of course it's a real big deal as we don't have that kind of money.