I doubt they ever really thought about it. Same thing with Giles never giving her compensation from his Watcher's salary. It's suggested in the show that Watchers take care of things in the cases of Kendra and Wood and possibly Faith before they were killed. But we're never told Giles does jack-all. He didn't help pay to get her tattoo removed or to fix Joyce's Jeep and we're apparently supposed to think him giving her some money in Life Serial was some magnanimous gesture.I’m always intrigued about what the writers were intending with this. Did they want us to be like hang on a minute why aren’t the other two adults in the house having to support financially? Or were we just not supposed to think about it?
Like it could have been easily sorted in a throwaway line with Willow saying something like they work a few shifts tutoring to bring in money but it’s not enough. Instead I get far too irrationally angry at everyone just staring at Buffy from the opposite side of the room with a stack of bills that haven’t been paid while we see nothing to suggest they’re even paying a few dollars rent