[Edit: I've been persuaded that I've been doing it wrong, so basically ignore this post. I'll do a new one.]
AD&D has rules for making magic items. They're a weird bunch and it's hard to follow the fragments, so here's an attempt at a guide based on what's in the books and some of my own interpretations. Although much of this interpretation is unorthodox, I believe it can be defended as "by the book" nonetheless:
1. Basic Wands
The simplest of magic items to make, a magician (6th level) can make a wand at a cost of 1gp per 100xp value of the wand to be made, and 1 day for each 100gp of this cost, or fraction thereof. Only wands which duplicate spells the character knows may be made.
So a wand of fear (3000xp value) costs 30gp and 1 days to make per charge. If you want 50 charges, it will cost you 1500gp and 15 days, up to the maximum of 3000gp and 30 days.
A wand must be initially made with the full complement of 100 charges.
Note that some wands (and staves) can not be recharged. These can not be made yet.
This is not normal spell casting as the character can do nothing else during the process, so I think the normal spells/day limit is not important here.
Scrolls can be made by enchanters (7th level) and above. These are quite variable in costs and in particular non-spell scrolls (ie, protection scrolls) can have very difficult ink requirements. The basic cost for "paper" is only 2 to 8 gp per scroll, and up to seven spell can be fit onto a single scroll. The time cost is one day per level of spell put onto the scroll.
For routine scrolls of spells, there is no guidance about what is a reasonable cost per spell, but I would incline towards 100gp/level of spell to match the other item creation rules. Otherwise, full details about scrolls and chances of failure etc are on DMG p117.
Potions are also added to the magic user's repertoire at 7th level, although from 7th to 10th level they will require the assistance of an alchemist. Only wizards and above can make them unaided.
The cost of a potion is its xp value in gp, and the time needed is 1 day per hundred gp or part thereof spent. There are additional overheads for the alchemist and his/her/its lab and potentially special ingredients which must be sourced as well as paid for.
At 11th level, if the wizard does not dispense with the alchemist's then time required is halved.
4 Basic Staves
Warlocks (8th level) can start to make staves, although if the staff has spell-like powers then the magic user must be able to cast the appropriate spells and if it has powers which can be used without charges, or which can not be recharged, it remains beyond a warlock's ability.
The process resembles that for wands but the gp cost of a staff is 4 per 100xp value of the staff per charge, to a maximum of 25 charges. So a Staff of Command (5000xp) with 1 charge will cost 200gp and take 2 days to make; one with 10 charges costs 2000gp and takes 20 days, and the full 25 charges cost 5000gp and 50 days.
As with wands, a staff must be made with the full 25 charges, although recharging need not bring the item up to full.
At this stage we've reached a break point in the ability of the magic user. All the items listed so far are "one-shots" - one charge is one use and when the charges are all used up the item is too (I assume scroll paper can not be recycled).
5 Enchant an Item alone
The additional items that become available with enchant an item at 12th level are those items which are not scrolls, potions, wands etc. but which have limited charges or uses, such as the chime of opening or Keoghtom's ointment.
In fact, almost any item of any sort (baring artefacts etc.) can be made if it is constructed in such a way as to only work once, such as a one-use ring of invisibility. The items' powers must still closely resemble spells that the maker knows, however.
A particular example is the ring of spell storing. A 12th level magic user can make such a ring and store spells in it. When those spells are cast, the ring can not be recharged as the spell storing was not permanent. Doing it "properly" requires the permanency spell. Once the ring is correctly done, it may be recharged simply by wearing it and "memorizing" the spells in question.
The gp cost of these items is simply their xp value and the time is one day per 100gp cost or part thereof. For cursed items (which have no xp value), use their gp value directly. The time requirements for enchant and item are additional to this standard cost.
When these items have used up their magic they are not destroyed, unlike potions, wands etc. but recharging them requires the full cost and time of the initial making.
6 Limited wish
At 14th level, the wizard potentially gains the use of limited wish. This may be used to charge normally un-rechargeable items and also put spell-like powers corresponding to spells the magic user does not know, providing that the spells are 7th level or lower (all at the cost of a year's aging). Additionally, items with 1 to 3 pluses may be made as well as items with abilities which do not resemble any particular spell but which the DM feels would be equivalent to 7th level or less.
The mage (16th level) can use permanency in conjunction with enchant an item to make most of the remaining magical items - those which have a continuing effect and do not need charges. At this point, most items are possible providing that they do not require 9th level spells or have spell-like powers equal to the 8th level or above.
Arch mages may make any of the remaining normal items using a wish to gain +4 to +6 to weapons, add special abilities which the DM equates to 8th or 9th level spells, or add bonus pluses or damage against special targets.
All items must be of high quality and possibly of special material. These factors are not included in the costs. So, a set of drums of panic may require (at the DM's fiat) to be made from the skin of a particular type of giant lizard. The PC must obtain this skin somehow and then treat it in some way. The treating cost and time is that which is given above, the obtaining of the skin may take any amount of time and money as the DM sees fit.
It is naturally impossible to make items which negate the cost of making them.
Similarly, assistance may be needed to make items such as swords and the more powerful the weapon the more difficult it will be to find a smith capable of helping.
Likewise, if an item is one which has a creature bound to it (eg, a djinn bottle), the magic user must obtain that creature as a special requirement of the making process.
If in doubt, the basic rule applies that cost is equal to xp value and time is 1 day per 100gp of cost. You may want to abstract special costs, such as the lizard skins example, into this formula.
Clerical and illusionist materials may be made using a similar set of guidelines along with some of the suggestions on DMG p118.