I guess I should rephrase that to "a fairly simple formula for me". But it might be easier to completely update the price-guide rather than to detail the program here.
Basically, I have the items in each class (Doom Arti, Doom Stealable, Il-marti, Tok, etc.) ranked by desirability from 1 (lowest) to 10 (highest). I use the pricing guide as my starting point and rank that price at a four on the scale. From there, each + on the scale adds 50% of the price guide value to the item value. Each - subrtacts 25%. So on this scale, an orniment of the magician on the price guide equals 30Mil, but it is on my desirability scale at 7, so current value is around 75 Mil.
The nice thing about this is that I do not really have to track the current prices by constantly shopping around, I just have to track how much I and/or my friends value that item over other items. If an item's drop-rate changes, I can slide it up or down the desirability scale easily.
This probable sounds confusing, but the system has worked well for me for the last few years. I may end up a little high or a little low, but I know I am at least in the ball-park.