I'd like to create set of Fascinating Daggers that will have a chance of casting Hypnotic Pattern or Hypnotism, something around once every 10 hits (I'd like it to tie it to the attack roll but not stack it with crits so probably on a 10 or 11).
Another way to view it would be creating a magic item with a use-activated spell and no space limitation (maybe a rune?). That leads to a 4k or 16k cost but I feel that there must be a heavy discount as it's random. Multiplying with the chance of activating it sounds reasonable although it could be argued that it should be even cheaper. This would give 400/1.6k. So for the lvl2 spell it seems ok to treat it as +1.
Using this formula, for a lvl3 spell and a 4/20 chance of activating, the cost would be close to a +2 (7200).
Thoughts? Is there something similar already?