I think that's a good rule of thumb for anywhere.
As a DM, I don't think you should be able to buy a +5 anything, or a +4 or a +3 or a +2, but maybe a +1 if you're in the mid-levels, say 8-14 and it would cost way too much to be worthwhile.
Actually, I don't even like the idea of PCs being able to get +4 and above (unless it is absolutely essential to the story). Very powerful NPCs might have these items, but they are the type of characters that are not likely to join a band of adventurers, or be defeated by them.
Just my $.02.
