When you get the artifact for 60 stars, it can "bring good luck to the owner" by not irritating Ms. LemonS when you get a question wrong, if you have enough stars on the star counter to pay his fee every time. I have seen his price range from 25-40 stars per assist, but I think even if you don't save enough stars by the boss battle, you will still get a 1-up by Neko. I prefer to buy him at the 5th and last round, since I hate it when he takes a lot of my stars away whenever I get a question wrong.