I have a few custom items that I’m trying to price, just in case the party wants to sell them and collect the gp/xp.
Bracers of Might [STR +2 (max: 18), may punch for 1d4 dmg, Dungeon Bashing proficiency] – this is a less powerful variant of Gauntlets of Ogre Power, which are normally priced at 50,000gp.
“Stars of the North” [short book containing various sky maps of the constellations - +1 to Navigation rolls in the immediate campaign region] – I have no idea how to price a +1 to a Proficiency roll
A spell that grants a proficiency, range touch, 6 turns duration, cancelable at will, arcane comes out to 15.6 points or 2nd level. Making a magic item that has this permanently active would have a base cost of 50,000 gp (500 * 2 * 50 for a permanent item with unlimited use). A proficiency stacked a second time gives you +4 to the roll. +1 being 1/4th of 4 (thank you, Captain Obvious), it seems reasonable to me to have the item cost roughly 1/4th as much, or 12,500 gp base price.
First one! The tables don’t have the numbers to straight make a spell for it. Eyeballing it, though, it’s a weaker version of Ogre Power, a 2nd level spell; let’s call it a first level spell. A continuous effect of a 1st level spell has a base price of 25,000 gp.
If you want to be particularly cheap, you can say the map allows you to use it once every hour, which with a duration of 6 turns is essentially constant. This would reduce the cost to 32% of the unlimited use. I’d say that only makes sense for a magic item that you’re going to have to “use” in the mundane way anyway, for the gauntlets I would say unlimited use is the more sensible costing.
If it’s not a magic book, then I’d say it’d be driven by mortal economics. A “Rare Book” is worth 150 GP if found as treasure. If this book wasn’t particularly rare, I’d make it cost less.
I did some more research. I think I should price the Navigation book like similar items, which can be bought as equipment. For example, a Holy Book that grants +1 to Theology rolls has a cost of 20 gold. Or, a Superior Instrument for 50 gold, which grants a +1 to Performance rolls. I can adjust for rarity as Susan suggested, but I don’t see it going for much more than 50gp.
