Ultimately, I find that 5e takes a much more 1e/2e approach to magic items than 3e or 4e did. That said, looking at the Buying a Magic Item downtime action (also in XGE), buying that moon-touched longsword would actually cost you anywhere from 120 to 170 gp (or more). I would always add the value of the weapon or armor into the price however, otherwise you end up with suits of adamantine and mithril plate that cost less than ordinary plate, which is just silly. I reserve the right to appear arbitrary when charging my players gold for magic items.Ĭommon magic items are actually 50-100 gp. That said, my magic item prices are more contextual, flexible, and gut-based than systematic. That determined player might expect to pay around 300 GP for something equivalent to a moon-touched sword, and 1500 or more for a +1 weapon. Like your table, there's no Magic Shoppe in my setting, but there are brokers who deal in magic items. I won't be throwing moon swords around my setting like confetti, but if a player is determined to acquire a magical weapon through market channels, I might produce something like this rather than a +1 long sword. But hey, it glows and cuts through Galeb Duhrs, so totes worth. Maybe it's associated with a clan of swordsmiths who use a special, magical infusion technique known only to family members, and they charge a premium price.
Maybe the ore used to make it is inherently magical and common enough in some remote part of the world, so it costs 3x as much as the normal version.
A moon themed weapon is generic enough to fit in almost any setting and can still have some interesting lore. I look at its inclusion in Xanathar's as an easy out for DMs who want to give sword lovers parity with other classes who deal magic damage, especially if +N magic weapons aren't ever going to be a thing. To answer the title question, I would treat such an item as worth more than 50GP in my setting.