I think the main reason there are no "official" prices for such items is mainly because it depends on the area you are in, the quality and size of the gem, the type of cut and maybe some other factors. Which can make the price be very different each time. In which case the custom input for them seems more appropriate.
That doesn't take away it could be a nice addition but I think there are still times you will need to pick the custom valuable option (and depending on the game/setting maybe each time).
That doesn't take away it could be a nice addition but I think there are still times you will need to pick the custom valuable option (and depending on the game/setting maybe each time).