Willow, and her male version (Warren), both could've become billionaires from their knowledge of radically advanced robotics even without magic, rubbing shoulders with David Nabbit (though magic could help make sure Willow got a fair deal). Heck, they way she's courted in high school (during career day) where recruiters flew in by jet say that others saw her potential, and they likely didn't realize she worked on Ted (can't remember if Ted happened by then or not).
If mercenary magic didn't work, W&H would be in trouble.
I'd say that thinking of making money through magic not being allowable is missing the point if they can't tell the difference between out of control greed (and exploiting others, which would get grouped with mind control magicks) and just taking care of loved ones (otherwise it would be evil to use magic to help with the crops and hunt and human fertility in the ancient days and so on). As long as money is the means to helping others rather than the end in of itself, then that should be okay.
On top of that, as someone who entertains children at birthday parties as one of my jobs (at least before the pandemic), I can just imagine how much joy she could bring to parties (which the parents would assume were tricks rather than real magic) that should be its own reward, and then using the money gratefully given to take care of Dawn (which means paying the bills in addition to all the things a teenager needs, and I'm sure maintaining the Buffybot and getting the urn wasn't cheap, either) is also helping rather than being greedy and selfish.
