Another factor in cost is the durability. The armor that was worth about 5,000 had a durability of 14 if I remember correctly while the $30,000 armor had around 40 durability or about 3 times more durability. You can see this in items that your repair. Because Geoff never has enough gold for me to sell my stuff to I let him fully repair my items. After the repair some of my items are worth $5000 more dollars.
Durability really adds up prices, My holy (Nobleman's) dagger had only 4 durability and in spite of having tons of level 5 stats (charm quality, mana drain, life drain, frost damage, poison damage, plus a bunch I can't remember), It was worth less than 4,000. Whereas when I got it with very little attributes (I loaded game afterwards, of course) but had tons of durability, it was worth about 50,000!
I found some gold ring that has absolutly no stats at all and its worth 25,000. So some of the prices are odd. I still have the ring only because the npc's in rivertown never have enough gold to buy my stuff. I always unload the havy items so I am not burdened and then the stuff they can't buy I either pile up outside the shop or give away.
Some items are like that. For example: Gauntlets of elemental mastery (no attributes, worth 47,000), Dwarven helmet of Warrior Priests (no attributes, worth 36,000) and others (like your gold ring)
It makes no sense to me why they have these useless itesm that are worth alot if the npc's have little gold to buy them off you. I also find lots of items with neagitive stats, like -6 to speed or -3 to sight. These worthless rings and amulets still sell for arounf $500. Kinda strange. <img src="/ubbthreads/images/graemlins/think.gif" alt="" />
Negative attributes really cut down the price of items. Geoff is the NPC with the most gold, he can get over 50,000 every couple of game days, so when he'd run out of gold I'd run around and fight until he got his gold back.