I have no reason to doubt there is a million dollars under that case. Their older, and much better, display clearly had one million in the form of 100 $10,000 bills. For those unfamiliar with them, $10,000 bills are extremely rare and sell for about ten times that at auction. Another reason I don't doubt they have a million dollars on the premises is every Nevada casino has to have sufficient cash to do business, and I imagine the Nevada Gaming Control Board lets Binion's count the money in that display, as a last resort. Ironically, not having sufficient cash on hand was the reason Binion's was closed down in 2004 (source).
To get back to your question, it would take 10,000 $100 bills to make a million dollars. Given that a bill is 6" long and 2.625" high, and a 100-bill stack is about 1/2" high, a million dollars would occupy 787.5 cubic inches only. That is just 46% of a cubic foot. You could fit a million dollars in $100 bills in a briefcase easily. So clearly there are some non-$100 bills in that case.
Discussion about this in my forum turned up an article with the specifics, Recurring currency from the Aug. 22, 2008 Las Vegas Review Journal. It says that the display has 42,000 $1 bills, 34,400 $20 bills, and 2,700 $100 bills.
This question was raised and discussed in the forum of my companion site Wizard of Vegas.