|"Jim Hawkins and the treasure of Treasure|
Island" - Georges Roux
From Wikimedia Commons
Referee could give some estimated value when players ask it and maybe even use intelligence score for that. Or you could roll the table!
Roll d20 and refer score to character's Intelligence. Only one character of the party can estimate the value for one item at a time. That rate is the final estimation.
Over INT - No idea.
Exactly the same - It is or is not valuable. Not sure how much though. Could be few gp or thousands.
5 lower - Roll d100. Results 01-49 is % lower of the real value. 50-100 is % above the real value.
10 lower - Roll 1d20. Result 1-10x10 is % lower of the real value. 11-20x10 is % higher than real value.
15 lower - Roll 1d20. Result 1-10 is % lower of real value. 11-20 is % higher than real value.
Natural 1 or 20 lower - Exactly the real value.
You could let player(s) roll the INT check and see the result or Referee could roll the die so players don't know how accurate the estimation is. If game has some lore or similar skill you can decrease it from INT roll.
Example: Bard has lore 2 and player rolls d20 result 15 and subtracts his lore value of 2 for final result of 13. His INT is 18 so he gets 5 lower and rolls d100 to see the estimated value.