I very rarely go for the €100 half hour. For me , one hour with a beautiful girl beats 2 for a half hour each, every time.
So I don't feel any need for a longer booking to be discounted. I often go for 2 hours, which again for me, beats 2 x 1 hours, and I've no problem with paying full rates for that either.
If you're going for 3 hours plus, it's nice to get a small discount as a gesture of appreciation for the business if nothing else.