Well since she's not even showing herself topless and always hides the nipple, but can still somehow sell some "hard nip pokies subtly visible through top" sets of 6 pictures and 60 seconds of video for $100 to droves of men and then do that infinitely, I'd say the 250-500k to show her tits is a laughable offer to her.
There's no point for her to go through all that effort, when the money is flowing in for no effort. Her and Sofie Gomez just have to put on some xmas girl outfit and stand next to a toilet for 1 whole minute turning a little, sell that with a slowmo on and ta-dah, thousands. Change to lingerie, sit on a bed doing nothing. Thousands. Go take a dip in a jacuzzi... THOUSANDS.
To guarantee her retirement when she's old and saggy, she only needs to put some actual nude topless shots from her prime into a safe drive and if there's ever a need for money, thousand upon thousands right there.
So I imagine HBO would have to pay her millions to show actual nudity even briefly, as it could only hamper the slow drip earner of "what if she shows a nip this time???"-dream the paying customers have. Something that would likely be gone to an extent if she showed the goods.