Never had it in me to ever ask a regular for a discounted price in case of being abused.
Same here. Especially if it's a girl I like enough to visit twice. Only "discount" I've gotten was a girl (she was my favourite girl before she disappeared tbf) who I had met on 3 occasions who asked if I'd photograph her and she'd give me a 2 hour session for free.
Felt like saying I'd pay HER to photograph her but kept my mouth shut. She had apparently come across a lot of guys offering photography for a price and they all turned out to be dodgy guys (one using his low-end iPhone for his equipment) and she said she trusted me and I'd previously talked to her about my photography in an arrangement. It all went well, it was basically a 3 hour session for free, can't complain. But then she disappeared from the face of the earth a couple of months ago and I have both her personal (which she later said she called me on accidentally) and her escort number and couldn't remember which was which so don't really want to bother her, especially on a personal number.
But in terms of asking a regular for a discount, I've never really had a proper regular (5+ meetings, I guess) but maybe if I felt confident and comfortable enough with her I might ask. But until she's at the point where she trusts you a lot and it's obviously a much lower risk meeting yourself than a random punter, then I can only really see it offending a girl.