The hope is that I can model other magic item prices using the same scrolls or stave formula by guesstimating equivalent levels of spells that would make up the effects. Let’s test out the idea with a few items. (Spoiler alert: it doesn’t work).
+2 Weapon: 2500
-A weapon specific bonus to attack and damage, like a fighter’s fighting style applied to both attack and damage. Usable continuously and infinitely. That puts it as a 2nd level ability, halfway between a 1st and 2nd level spell slot equivalent. 40*12+40*1.5*12 = 1200, about half too low, but I’m not sure if the stacking sits right. This doesn’t account for the fact that it bypasses many damage resistances now and is harder to damage.
Alternatively, at 10th level a champion fighter gets a second fighting style, so that could put this as a 5th level spell, usable continuously. 1000*12 = 12000. Way too high.
If we used the Magic Weapon spell, cast using a 4th level spell slot, this would cost 400*12=4800. Still too high, but the spell does last for an hour so it might be closer to 400*10 = 4000. Add in the limitation that it is stuck to one weapon permanently, and you could edge a bit closer maybe.
Ring of Backstabbing 4500
-Bonuses to sneak attack equivalent blows, like a 3rd or 4th level rogue, but it also includes a minor attack bonus as well. Usable continuously and infinitely. If we call this two 2nd-level spell equivalents (attack bonus and sneak attack damage bonus) that would come out to 60*12+60*12*1.5 = 1800. Even if we bump this by 1.5 to account for the fact that it is on a ring rather than a more restrictive piece of equipment, we still only reach 2700. This is just over half.
Alternatively, if we treated this like a 5th level rogue when they reach 3d6 sneak attack damage, to match the +2d6 sneak attack damage dice of the ring. A 3rd level spell used continuosly should be 150*12=1800 again.
A 4th level spell slot wand would cost 4800 for continuous use, but that is a bit too high, and I would definitely take a 4th level spell over the restricted +2d6 weapon damage.
Cloak of Resistance 250
-A small boost to the wearer’s save’s as if they were a lucky halfling. Usable continuously and infinitely. Call this one a 0th level spell and it is worth 10*12=120. Double that for a continuous effect requiring no action? That puts it right on par.
Similarly, if we treat this as a 1st level ability, it would be worth 300 instead.
It seems the difficulty here is that while we could backtrack and reverse engineer a plausible reason for an item costing what it does, I don’t see an easy way to classify magic bonuses by an equivalent spell level. They depend greatly on the underlying number they are improving. A magic weapon is much more powerful in the hands of someone with many attacks than someone who makes a single sneak attack each round. Multiple weapons are stronger in the hands of a rogue who can hedge their bets to ensure they get a successful hit. Armor class bonuses are stronger the more of them stacked together, same with saving throws, statistic bonuses, and skill checks. But none of these things are easily translated into differential prices, at least not without breaking game immersion. Spells however, are often just spells. They rarely grant numeric bonuses and those that do don’t tend to stack with other sources of bonuses. The most important problem though, is that items and spells tend to be powered relative to different categories. Spells have an innate relative power by spell level and to a lesser extent by which classes can cast them. Scrolls and wand costs come from that to try find the balance between scrolls and wands so cheap they become ever present or so expensive they are never used. Magic items, on the other hand, are one of the default ways that money is spent and characters progressed. Their worth is determined solely relative to the other ways that gold could be spent or other treasures that could have been sought.
Trying to reverse-engineer item prices from their equivalent spell effects or spell level is doomed to fail. It becomes a self referential loop. My original drive in creating a spell level, wand, scroll, and spellbook formula was to generate a rough value for those stored spells relative to other magic items that players could purchase. By attempting to turn back around and tweak item prices or even classify them by spell level would make it a circular reference, detaching it from any grounding it had. An interesting thought experiment, but ultimately, I think I will let this idea go.