I made the mistake of getting all the available stars in Super Mario Galaxy before facing Bowser for the first time. This leaves only the purple comet stars in the post-game, the problem being that, because purple comets are prankster comets, they disappear after you collect a star. The game wants you to pay your hard-earned star bits to make them re-appear, but what if we could make those pesky purple comets re-appear without funding the luma monopoly? A lesson in Luma tax evasion.
The 'official' way
The correct way in the game to make purple comets re-appear is to head to the Comet Tico on the lower left deck of the Comet Observatory and pay the lil' entrepreneur no less than twenty starbits to allow you to progress in the game. Life in the milky way is terribly expensive!
The frugal way
Luckily, the universe is on our side. If you collect any star, even one you've already collected, the game will reinstate the purple comets just the same as the luma will, only for free! The challenge is to make the time you spend on collecting that star as short as possible. I've found that Luigi on the Roof in the Good Egg Galaxy is the best candidate. Since Luigi is on the roof of the house in the starting area, it only takes a well-timed triple jump or a backflip to reach him and collect the star. In under a minute you will have your purple comets back without having contributed to intergalactical fraud! Happy star collecting!