I think what's basically being said is that the Balanced Dagger is the item version of WONAFYT: If you don't find it very early (at level 1, with minimal exploration, preferably before your first kill) it's pretty much useless. Is there a way to increase the chance of it being in the store closest to where the player spawns? Then an 18 cost might be acceptable. It's an item with a very narrow lifespan, because 1) it's never better than a Yendor you can afford/steal to get, 2) it's never better than fighting anything two or more levels higher, so even if you get it at level 1, if by level 3 you can kill 5's, it's already lost it's useful life after 4 kills, and 3) at 15-18 gold, you're giving up a Schadenfreude potion, Strength potion, Helm of Heroes, Whurrgarbl, or similarly strong item, or in many levels, putting yourself just out of range of things like Dwarven Gauntlets + Vampiric Sword (or affording Yendor itself!)
It's in some cases very handy, especially for classes that can have trouble killing above their level, particularly on vicious - which leaves it in the unfortunate spot of being the kind of thing you'd be best off using as a preparation, but being far too narrow to give up a precious locker slot for. I'd have to have something like 10 to 12 lockers before I could spare a space for something I'd only prep for things like a Vampire in VHoS or something.
I mean if it's intended to be a very narrow item, then that's fine, but if it's going to be that narrow and people aren't using it as much as you'd expect, those are probably the reasons why. Even under optimal conditions of finding it, it's only good in most cases for an extremely narrow time span and somewhat pricey for that. Compare to the Bloody Sigil which is also useful only at low levels, but costs 8 and converts for 45.