Most of my players are magic users, so I created a homebrew wondrous magic item that I hope they will find slightly useful and mostly enjoyable for many game sessions to come (they are mostly level 6 or 7). I know how to figure out most properties of the item, but I would like to know how to figure out the price.
Here's the item:
Novel of Syntactical Magical Novelties
This small leatherbound and shabby-looking booklet contains one dozen worn and torn yellow pages with random scribbles on them that don’t seem to make any sense to the casual observer. Upon examination, magical writings appear to be hidden amongst the scribbles.
The booklet always contains exactly one spell attuned to the owners magical preference and usable directly from the booklet once a day by the owner without requiring any spell slot. The spell is always a syntactically incorrect version of an existing spell and modified to behave accordingly (for example, Wind of Guts instead of Gust of Wind might add a nauseating effect). Casting a spell from the booklet requires the owner to be familiar with the spell and requires perusing the booklet which results in a full round of casting in addition to the normal casting time of the spell. The spell cannot be memorized or copied out of the book in any way and is replaced with a new spell at the start of each game session (by the DM). Transfer of ownership of the booklet won’t be honored until the next game session (the booklet, its contents and its limitations on daily use remain unaffected).
Caster Level is probably 17th because I haven't included an upper limit on the spell that could be in the booklet (although if I would also allow metamagic Caster Level is probably even higher?). Price probably varies? I have no clue how to assess that because almost any (metamagically enhanced) spell could be in the booklet (as long as it fits on 12 pages). or should I maybe consider pricing it as a regular 12 page booklet because to most common shopkeepers it will probably seem useless?
PS: I'm only interested in the sales price; item creation price doesn't bother me because I won't allow any copies of the book.