She got a very small payoff, relative to royal life.
The Lownie book states that she completely left it to the palace to dictate how much was enough and they took full advantage of this.
I don’t excuse her other unbelievable and disgusting behaviour in any way at all but this was the one time she wasn’t money grabbing.
Apparently the palace was only prepared to offer her a two bedroom cottage on the Windsor estate with an outside bathroom which sounds extreme to me but given how careful Lownie has been in his research in general I believe him.
She therefore declined and rented a much bigger house.