Used to mean pricey rather than affectionate. You drop it when something feels like a bit of a rip-off, usually with a wince and a look at your mate like, are they having a laugh. Works for anything from pints to rent, and it can be mild or proper annoyed depending on the tone. Classic British understatement.
"Six fifty for a tiny bacon roll at the station and they still want 50p for sauce. Bit dear, mate, I’ll pass."