The basic rule in stacking magical effects on magical of items is this: The cheapest effect has the normal market price, that of other effects is half again as much.
First a quick definition of terms: magic items have a cost and a market price. Cost is the what it takes (in gold pieces) to build the magic item. Market price (or just price) is like a manufacturer’s suggested retail price—what it should actually cost to buy. In the Crimson Plains and surrounding regions, the Veneficus Professio Ordo regulates the price and sale of magic items, so these market prices are prices set by the Order. (This is mainly to give a game world and roleplaying reason why magic items have fixed prices—otherwise, a magic item price would pretty much bring what the market could bear.)
For example, let's say you have a headband of intellect +4, the item that grants a +4 enhancement bonus to Intelligence. Its base market price is (like for all items that grant enhancement bonuses to an attribute) is the bonus granted, squared (that is, the bonus granted multplied by itself) times 1000, in gold pieces. The market price of a headband of intellect +4, therefore, is 4 × 4 × 1000, or 16,000 gold pieces.
Now, let’s say that you decide you want your headband of intellect +4 to also act as a circlet of persuasion, an item that adds +2 to all Charisma checks and Charisma-based skill checks. The market price of a circlet of persuasion is 4,500 gold pieces, so it is the cheapest of the two enchantments. That means that the headband of intellect +4 part of the enchantment now costs half again as much, so the market price of the item is 16,000 × 1.5 + 4,500, or 28,500. Since 16,000 of that has already been paid, it is subtracted from the price, so the cost of enchanting an already purchased headband of intellect +4 to also act as a circlet of persuasion is 28,500 – 16,000, or 12,500 gold pieces.
As for the time to make items, that is simply the market price divided by 1000, rounded up, in days—except for a potion, which always costs a day to make no matter how expensive. (There are feats that can speed this up.) So it takes 16 days to make a headband of intellect +4, 29 days to make a circlet of persuasion combined with a headband of intellect +4, and 13 days to make a headband of intellect +4 into a circlet of persuasion combined with a headband of intellect +4.
Realize, of course, that what I have described here are the guidelines for pricing magic items. The actual rule, as one Sage Advice answer went, is “as much as players will pay for it, and not one copper more”. As they put it, if your players sell it as soon as they know its market price and buy cheaper but more useful items, it’s probably too expensive. If they are lining up at the archmage’s tower to buy a dozen, it’s probably too cheap. That’s why some items bend or ignore these guidelines. Because mobility is so important in combat in Dungeons and Dragons®, objects that grant flight are very powerful and saw a big increase in market price (or reduction in effect) between version 3.0 and 3.5. Likewise, items granting a haste effect were nerfed along with the haste spell itself.
“You are an artist,” Yeste said.
“No. Not yet. A craftsman only. But I dream to be an artist.”
—Domingo Montoya,
The Princess Bride, William Goldman