Yes, it's all about decency. When you take something off a rack in a Woolies, that's a commitment there to buy that product, you gotta pay the price even if you forgot to check the price tag.
A handful of times I got into really passionate sessions where there was no opportunity to talk about price. It became like making out with a GF, you don't talk $$$ in those situations.
What I do at the end of the session depends on the girl. If she's not very business-like, I just pay a rate that I know would make her happy, usually a fair bit more than the going rate. Some girls have returned part of the money to me when I did this saying I'm overpaying.
If she's business-like or a more experienced type, I just ask how much I owe her and pay up.