If we assume they maybe averages 75% of that over 6 hours, that's 146Gb/s * (60606)s / 8bytes/byte * 0.75 * 0.05dolars/GB = $14,782.50 for a 6 hour period (estimate).
That's highest listed bandwidth tier price for Azure, I'm sure they would pay less for a number of reasons. But let's just say that $10k for a whole day may not be out of the realm of possibility. That puts bandwidth costs possibly North of $3 million a year. Even at $1 million, that's a lot of money.
And if that's audio/video Vs just text, clever NAT hole punching techniques could reduce it if truly needed.
But, it seems WhatsApp was on SoftLayer? In that case their costs might have been vastly higher.
[1] http://web.mit.edu/e-club/hadzima/how-much-does-an-employee-... (This is old but nothing really substantial has changed except.)