Sometimes, particularly during city adventures, you have to buy and sell alotta items, and you don't want to look them up (or they're not on any equipment list you own). Like if somebody wants to buy a quick disguise to enact a weird scheme and you want to wrap up this part of the adventure before dinner and you don't really want to spend a half hour shopping.
This is a formalization how I generally think of it--Penny, Nickel, Dime, Quarter, Dollar:
"Penny" items are things that cost less than a G.P.. I assume all the things an average medieval joe might buy in an average medieval day all add up to about 1 gp. So you can get a humble meal or two, some nails, a torch, some thread, and a "dose" or three of beer for less than a gp. Anything on the daily shopping list. Food animals count as if they were the number of days food they represent--like if you think a chicken is 4 days food, then it's about 4 gp.
If you need to know exactly how much a sub-1 gp item costs (4 cp? 24 sp?), stop using this system and look it up, lazy ass.
"Nickel" items are anything you'd take camping and basic adventure gear. Rope, pole, spikes, lantern, etc. 5 gp per syllable ("lantern" is 2 syllables--10 gp).
The syllable thing sounds silly but more syllables generally indicates the PC wants a more specific thing (i.e. not just "rope" but "silk rope" not just "a lantern" but "a hooded lantern"). If you ask for something general you're going to get the humblest item that qualifies--like if you ask for a "horse" you won't get a warhorse, and if you ask for a "warhorse" you won't get a heavy warhorse.
"Dime" items are specialist items--anything that usually only a certain profession or class would use. Thieves tools, navigation tools, a bible, a marionette. Dime items cost 10 x number of syllables in the name. So: a lute costs 10 gp, a cello would cost 20.
"Quarter" items are luxury items. A string of pearls, fancy shoes, etc. 25 gp x number of syllables in the name. A "Rich old woman's clothes" would be 125 gp. (You'll notice women's clothes always cost 25 gp more than men's.)
"Dollar" items are things which are lethal or highly dangerous all by themselves--drugs and dangerous animals included. 100 gp per syllable--poison and acid would be 200 gp per dose, a wardog or falcon would be 200 gp, gunpowder or a heavy warhorse would be 300 gp, etc.
Weapons: Melee weapons cost gp = maximum normal damage. Missile weapons cost twice that.
Armor: Armor? Seriously? Look it up.
So, yeah, that's that. All kinds of things are unrealistically pricey or cheap when you do this, but remember, this is just for when you're trying to get things to move fast. If you have time to look up items, do it.