You would probably be better suited to asking this on reddit as this is not a Roll20 specific question and more of a general D&D 5e (?) question.
However, it depends on your game style. Some DMs just tell the players right away how much these items are worth because they don't want to have to remember it themselves. Others make the characters roll for it, you can base it off a character's Intelligence check or maybe even add in a tool proficiency. For example proficiency with a jewelers kit would allow the character to better approximate it's value (I would even rule that if the character had proficiency with the jeweler's kit as well as actually had the jeweler's kit in their possession that they would automatically know the value of gems). Art objects for me would be a different proficiency depending on what the art object is. Maybe for paintings I would require proficiency in the painter's supplies.