I don't have the price for the spellbooks, but I assume it would be fairly inexpensive, compared to other spellbooks containing spells. I also assume that most mage types wouldn't be caught dead with a book like that, and many deities would be most displeased if a follower tried to use one. Various dour types may not be able to use joke spells at all, since they don't have enough humor to power the spells.
This is one of my favorite house rule additions of all time. And I say this as a humorless curmudgeon who had no tolerance for gnomes until demanded by a backer. When the time is right I would love to include this in AXIOMS.
I also think you're on to something with regards to 0.5 spell points being a useful demarcation for 0 level spells. I could see the Prestidigation proficiency being replaced with an opportunity to know some X number of 0th level spells which can be cast at will or some such.