I think the issue isn't in not being able to pay, but with deciding the threshold at which you're not able to pay.
The lower your humanity is, the closer you are to being an animal and animals don't conform to societal arrangements. At this point you - as a player - can't PAY a hooker, because you - as a character - don't see the need to pay them. You can simply TAKE blood (by force) from whomever you want whenever you want. It also makes sense if we take a look at this in the context of the Masquerade itself: you need to have high enough humanity in order to be able (and willing!) to take the blood covertly. Because, let's face it, vampires don't really hire hookers to fuck them. They hire them for blood. And by paying a hooker you ensure easier and more secure access to food, both of which are important for the setting.
Take a look for the scale from
Vampire: the Masquerade - Bloodlines:
Mechanics
A Kindred's Humanity is measured with a scale, ranging from 0 to 10.
0: When a vampire's humanity reaches 0, the vampire is completely lost to the Beast, so deeply that it is impossible for him/her to take control ever again. Such a vampire is called a Wight, and the act of reaching 0 is called Wassail.
1-4: They are vampires who will do any heinous deed and will gain pleasure from it. No crime is horrendous for them, as they delight in their bestial nature.
5-6: They will usually be distant and neutral, as they would not see the need for excessive murder, but they would not see the need for protecting life either.
7: They have the same feelings as in life, with the moralities and taboos of humans.
8-9: These vampires are extremely compassionate, as they will fight every night against their Bestial urges while trying to protect humans as well.
10: A 10 is viewed as a saint in the world of the Kindred. They are completely repulsed by their nature and they follow an incredibly strict moral code in order to keep themselves out of the Beast's reach.