APPRAISE (INT)Check: You can appraise common or well-known objects with a DC 12 Appraise check. Failure means that you estimate the value at 50% to 150% (2d6+3 times 10%,) of its actual value.Appraising a rare or exotic item requires a successful check against DC 15, 20, or higher. If the check is successful, you estimate the value correctly; failure means you cannot estimate the item’s value.A magnifying glass gives you a +2 circumstance bonus on Appraise checks involving any item that is small or highly detailed, such as a gem. A merchant’s scale gives you a +2 circumstance bonus on Appraise checks involving any items that are valued by weight, including anything made of precious metals. These bonuses stack.Action: Appraising an item takes 1 minute (ten consecutive full-round actions).Try Again: No. You cannot try again on the same object, regardless of success. (SRD Appraise)
I never have my players roll for things that they should automatically succeed at doing.
Instead of having my players rolling for common items I only have them roll for uncommon items such as gems, jewel encrusted weaponry, artistic objects, magical items, and the like. This is a subjective list of items as certain characters can become intimately familiar with uncommon items through repeated exposure to them. For example, the Rogue can become very familiar with the value of small gems 2 to 4 Carats in weight while his Fighter companion might never come into enough contact with small gems to get beyond a ballpark value for the things.
When it comes to modifying the chances for success the SRD tells us that a magnifying glass and merchant's scales can give our characters an additional 4 point bonus to the check; but there are additional ways to increase a character's chances for success. To determine what sort of things could increase a character's effectiveness let's keep things in the realm of logic, with only occasional touches in the fantastic.
Familiarity with the component parts of an item increases the chances that a character would get closer to the actual value of an item, for example, a Rogue would be able to asses the values of the gems on a golden, bejeweled mace. Additional aid from other characters who are more familiar with the base materials and workmanship could also help us, for example, a Dwarf is intimately familiar with the values of precious metals and could give us a value for the gold shaft of the bejeweled mace.
By getting help from the other players we can easily increase the bonuses for our character within the confines of the base game and can push the circumstance bonuses up to an additional 8 point bonus.
I'm okay with the Dwarf's racial bonus but the raven bonus doesn't make a lot of sense. As a species raven's are fond of shiny objects but not of valuable objects. Tinsel, pretty ribbons, and colorful string are as likely as gold to be sought after by the raven.Special: A dwarf gets a +2 racial bonus on Appraise checks that are related to stone or metal items because dwarves are familiar with valuable items of all kinds (especially those made of stone or metal).The master of a raven familiar gains a +3 bonus on Appraise checks.A character with the Diligent feat gets a +2 bonus on Appraise checks.(SRD Appraise)
I wish that I knew why they gave this bonus.
With uncommon items I tend to set my baseline DC at 15. This creates a situation where these items can have their values discerned by your average character without establishing a DC that is too high for all but the most specialized of characters.
For rare items I tend to establish my DC between 20 - 25 to reflect both their rareness and the notion that specialized characters would be the only people who would be able to successfully determine their values.
Now we can continue talking about ever increasing DCs but the truth of the matter is that after a difficulty class of 25 the characters who can make that check are going to make it regardless of what you do as a Dungeon Master.