Rough math wise and ATM can easily hold $40k. That’s 8.6 inches of $20 bills, it might be a mostly full ATM, but a believable amount.

As for how easy it is to break into an ATM, it’s not easy, but it’s not impossible. They are designed to be serviceable by a single person, so the doors and locks while strong, aren’t going to hold up to the force of a vehicle or explosives from someone that knows what they are doing. The hard part is removing the cash box, which depending on the machine it can be easier to take the whole machine. Once you get the machine/box to a second location the security is irrelevant.