In the campaign I am running, I have designed the world where there are a certain type of monster that comes out only at night. They like to destroy and hurt things, (not simply kill them, but it happens too). It makes travel at night much harder. Towns and cities are warded against these things, but I imagine that the party will want to leave town and travel at night. I have guides and traders that do this, but they carry with them portable magic circles against these things made out of metal slats and connected by silver chains and covered in magic runes. I'm trying to figure out how much, according to the math, they should cost. They are supposed to be nearly impossible to find and extremely difficult to make, as it is hard to find someone who can write in the secret rune-language.
Thoughts?
If they are nearly impossible to find or create, they likely won't have a normalized price. Such an item would be of incredible value due to its rarity and obvious use.
When in doubt, though, I would start from the stats for pricing Magic Circle Against Evil as a continuous effect and work from there.
Treat them almost like a (very) minor artefact. These things sound so necessary that selling one is not going to happen often or cheaply, and the method of creation is so difficult/rare that it is borderline forgotten.
Seems to be close to the vibe you're going for. If you try and price it as a normal item, it is probably too expensive to justify.
Continuous use, slotless wondrous item = (spell level 3 x caster level 5 x 10 min/lvl duration 1.5 x 2000 x 2) = 90,000gp
You can reduce that with an ad hoc reduction for only usable at night, or leaving off the x2 slotless modifier - creating magic items is as much art as science :)
I have guides and traders that do this, but they carry with them portable magic circles against these things made out of metal slats and connected by silver chains and covered in magic runes.
Instead of pricing for the item, price for the guide...