Expanding the Equipment Lists (DMG)

The items listed in the Player's Handbook are by no means the only things ever made in the world—or even in a medieval fantasy setting. They are listed because they are the most likely things the characters will need. However, you can certainly add missing or player-requested items to this list.
When you add an item to the lists, first consider the reasonableness of its presence. Given the setting of the AD&D game, adding an M4 Sherman tank as a regular item of equipment is just not a logical, sensible, or wise thing to do.
Once you decide that a new item is reasonable, you must assign it a cost. Use your judgment. Consider the intricacy of the item, the craftsmanship required to make it, and the cost of similar items already on the lists. From these, you should be able to assign an appropriate price.
If, later on, you discover you made the item too cheap and all the characters are buying one, raise the price and say suppliers can't keep up with the demand. If an item is too expensive, you can lower the price and no one will complain.