How do you figure Market Value?
That's really the trick, isn't it? Some days I look at Table 8-40 on page 242 of the DMG and wish it wasn't there at all. At these times, I wish the rule was simply, “Match your new item as closely as you can with an existing item, then give it a similar price.” That's really the ultimate pricing rule. It can get you into trouble (as it did with me and boots of striding and springing), but generally it will give you fewer headaches than using the table. At the very least, we should have called the table "Estimating Magic Item Gold Piece Values" rather than "Calculating Magic Item Gold Piece Values."
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The most important thing to remember is, Table 8-40 doesn't determine prices. It suggests them. Don't say, "Wow, these shoes of continual improved invisibility sure are cheap." Do say, "Hmm, these formulas don't work when it comes to spells like improved invisibility." When someone asks me, "Can I really make an item that will cast cure light wounds at will, activated by a command word, for only 900 gp?"I now reply, "Only if your DM isn't paying attention."