For the past few years, I have adapted a technique elaborated by Zak S. in his Vornheim supplement for setting the basic prices of goods. Here, I want to discuss the technique, some expansions and adaptation I've made to it, and some alternative techniques I also deploy to resolve the question of "How much do things cost?" and "Can I get a deal?"
Zak's technique is to calculate the cost of an item as a number of silver pieces (in a silver standard economy) equal to the # of letters used in its name and description. "Rope" costs 4 sp, for example. This runs into a bit of a wonky situation with the way plurals work in English, so I generally ask people to specify quantities (getting ten of something for slightly cheaper than the cost of nine is treated a bulk discount).
The first expansion of this technique I've introduced and experimented with is to modify the cost of the item through the application of negative adjectives. The negative adjectives reduce the value of the item by the # of letters in them. If they would reduce the cost of the item to zero or less, instead reduce the type of currency down one step (so from silver to copper) and then continue subtracting copper pieces based on the remaining unused letters in the negative adjectives. If you bottom out of copper, well good luck, it's trashed.
You can, of course, fix up or repair an item to remove negative adjectives, restoring its value.
The second expansion involves using this to haggle. Each side gets one or two passes at describing the object, trying to tack on adjectives describing its quality, value, etc. to boost or diminish its price. I typically allow three adjectives to be added per pass. Anything they say about the object becomes true, but you can't fundamentally change the object, so no taking this "jade ornament" and saying "It's actually a katana".
I usually make the NPCs go first, partly to give the players an idea of how long a pass is and what kinds of claims you can make about items during one. You should encourage the PCs to write down the specific adjectives they're asserting, and write down your own in turn. The fun of this can evaporate and it can be tedious to track everything after more than two passes on each side, and after four statements of three adjectives each, you have twelve modifiers (Try this: Write the adjectives in matching pairs from each side on grid paper with one letter per square. Cross off letters in each instance until you're left with a subtotal for each side, then subtract the smaller subtotal from the larger to arrive at the actual price difference).
This whole process works well when you're buying or selling individual items, especially if they're high-value treasure like magic items (and you can slide in plot-hooks about them through careful preparation for your descriptions as well).
One minor but important point is that once PCs decide to do this, they are locked into the sale or purchase - no backing out. The assumption is that this represents the best price they're able to get for the item.
For treasure items that don't have a clear value because they're made of precious materials, magical, or whatever, I use this technique to quickly generate their value.
For large bulk purchases or sales of mundane items, like when the PCs are resupplying their mundane gear; equipping a caravan, ship or castle; or engaging in the speculative trading of bulk goods, I just roll (2d6+3) x 10% x list price. That gives you a range between 50% and 150%, with the majority (2/3rds) of all results between 80% and 120%.
I got this trick from Mongoose Traveller, which uses 3d6 x 10% to calculate the sale price of bulk goods. But having played around with it a bit, I think the extremes are a little more extreme than I normally want, except in a game where the PCs are merchant-traders trying to get rich and so those rare purchases at 30% of list price and 180% sales are huge wins for them (that is, when playing Traveller). And conversely, sales at 30% of value and purchases at 180% are really disincentives to get rid of stuff, disincentives that stop the conversion of treasure or goods into cash entirely. A system for pricing that set them so that people don't want to buy and sell is not a good pricing system.
I also only roll the 2d6+3 once per visit to town, instead of doing it for individual transactions or shops. It covers both sales and purchases simultaneously - if you're in a market undergoing inflation, you're also taking advantage of that inflation when you sell things, and vice versa with deflation. This means you only have to do the calculations once (I use an Excel spreadsheet when I have a computer handy) instead of repeatedly.
If PCs are unhappy with market conditions, then I provide them with two options, one of which they can engage with prior to going on the market, and the other they can engage afterwards. First, for specialty treasure, they can use the haggling procedure mentioned above. Second, along their adventures, I try to have them encounter and form relationships with merchants. If they have a special relationship with someone, then they can make a single new roll just covering things they're buying or selling from that merchant, and take the better of the two for their purposes.
I roll the market conditions after the specialty items are dealt with because it speeds things up by removing the comparison between the market prices vs. possible haggled prices prior to decision-making.
e.g. The PCs in one of my Necrocarcerus campaigns once saved a gas station clerk from being transformed into a skeletal warrior by a fire cult / undead rights organisation. This would count as a special relationship that would allow the reroll if they tried to buy gasoline or other supplies from his gas station.
The order of procedures for integrating all of this in my games is as follows:
1) PCs arrive in town and announce they want to buy / sell supplies and treasure.
2) Magical and specialty treasure is handled first. PCs haggle with merchants over the prices and the sales are concluded as described above.
3) Roll (2d6+3) x 10% and calculate prices for mundane and bulk goods.
4) PCs draw up a list of goods they want to sell or buy w/o fuss or muss. The modified prices are applied. If there are specialty items the PCs didn't sell via haggling, they may now sell them as well, at market prices.
4a) PCs may withold some items to sell at special merchants they have relationships with. If so, they may reroll for those merchants and recalculate the prices then make their sales or purchases.