This is from the 1st edition DMG, you could try some variant of this one.
Gold Pieces: Convert all metal and gems and jewelry to a total value in gold pieces. If the relative value of the monster(s) or guardian device fought equals or exceeds that of the party which took the treasure, experience is awarded on a 1 for 1 basis. If the guardian(s) was relatively weaker, award experience on a 5 g.p. to 4 x.P., 3 to 2,2 to 1,3 to 1, or even 4 or more to 1 basis according to the relative strengths. For example, if a 10th level magic-user takes 1,OOO g.p. from 10 kobolds, the relative strengths are about 20 to 1 in favor of the magic-user. (Such strength comparisons are subjective and must be based upon the degree of challenge the Dungeon Master had the monster(s) pose the treasuretaker.)
Treasure must be physically taken out of the dungeon or lair and turned into a transportable medium or stored in the player's stronghold to be counted for experience points.
All items (including magic) or creatures sold for gold pieces prior to the awarding of experience points for an adventure must be considered as treasure taken, and the gold pieces received for the sale add to the total treasure taken. (Those magic items not sold gain only a relatively small amount of experience points, for their value is in their usage.)