I'm going on some very rough maths here based on a few estimations I've made for celestial items (helm/neck/brace/ring/boots/belt) and for weapons/armour, given personal experience and first hand information on things that have been dropped, how many of each mob are actually killed, and a healthy dose of extrapolation. They may not be accurate, but I'd say as estimates they will be good enough
I think for mobs in CT (normal mobs) you're about 1/200,000 for items and 1/2,000,000 for weapons/armour (bear in mind these rates are greatly improved for super/champion), subs (guardians) something like 1/5000 for items and 1/20,000 for weapons/armour
Tomb of the Dead your normal mobs will be a lot lower rate, I'd estimate in the region of between a 5th and a 10th of the rate of CT, and biles 1/20,000 for items and 1/80,000 for weapons/armour
So for example, if CT mobs have a rate of 1/200,000 for each item and there's 6 different types of items, that means you're looking at 6/200,000 chance each mob, so on average you should see 1 cele item in around 33k mob kills, which approximately equates to clearing all the rooms 16 times.
As I said, my estimates may be a bit optimistic, but if the drop rates were any higher there would be too many items and they'd seem less special, some people really do kill bonkers amounts of these mobs. When solo I can easily do 1k+ kills per hour and there's loads of people who can do the same, on a big exp recall group hunt we probably do more than double that rate
To conclude, a billion to 1 chance of a drop is maybe a little higher than reality, but not too far off the mark in terms of scale