Q: How much should this magic item I’ve designed cost?
--Too Many Questioners to List
A: As much as your players are willing to pay for it, and not a copper piece less.
Setting the price of a newly designed magic item is as much art as science. Compare the potency of your new item to other items in the game (particularly those that you know your players like to use). Find an item that’s about as useful and powerful as yours, and set the price there.
If your players immediately sell it off after finding it in a hoard and learning its gp value, you’ve probably set it as too expensive. (As a general rule, any item worth more than one-third of an average character’s wealth at a given level is probably too expensive for him to keep unless it’s exactly what he wants; he’s better off selling it and buying a weaker, but more useful, item.)
If your players line up at the local archmage’s tower to have him make a gross of the new item, you’ve almost certainly set its price too low. (Again, at a certain point an item’s price is meaningless compared to a character’s wealth. If your item costs less than 10% of an average PC’s wealth but it’s still useful, be ready for everyone to have one—see the wand of cure light wounds as an example).
When in doubt, set the price of an item that you’re making available for purchase higher than you think is correct. You can always drop the price later if no one buys it for a level or two.
Conversely, when putting an item into a treasure hoard, set its price lower than you think is correct. If the players keep it and run out to buy more, tell them that only one is available; then set the price for the next one they buy has gone up by 50% and see if they’re still buying. Continue adjusting as necessary until you find the right spot.