Prices for magic items and other very expensive goods being the same in almost every setting isn't (just) a simplification to avoid putting local price fluctuation mechanics in the DMG. Rather, all of the major metropolises on the various Primes are connected by enough spelljammer routes, portals, teleportation circles, and similar, and the major trading groups (mercanes, the Factions, Prime guilds, etc.) have access to sufficient divinations, to ensure that the multiversal magic item trade has essentially optimal prices.

For instance, if every magic item merchant in Waterdeep decided to collude to raise prices on, say, cloaks of resistance +4, any high-level adventurer who wanted one could pop on over to Sigil, Union, Dis, the City of Brass, Greyhawk, the Rock of Bral, Sharn.... and pick one up at the standard price; if a new vein of adamantine is found on a particular Prime, planar traders can find out about it and add it to their list of thousands of adamantine sources throughout the multiverse; and so forth.

In more humorous campaigns, there may be an explicit mercantile council overseeing pricing policies, the Wizardry, Artifice, Logistics, Magic, And Retail Teleportation Company, or WALMART Co.