From the OD&D FAQ in Strategic Review #2:
Valuable metals and stones, however, are awarded experience points on a 1 gold piece to 1 experience point ratio, adjusted for circumstances—as explained in D & D, a 10th level fighter cannot roust a bunch of kobolds and expect to gain anything but about 1/10th experience unless the number of the kobolds and the circumstances of the combat were such as to seriously challenge the fighter and actually jeopardize his life.
It was always clear that experience awarded for defeating monsters should be adjusted downward for monsters significantly weaker than the player characters, but this seems to suggest that the experience for treasure should likewise be reduced for treasure acquired too easily. Page 85 of the DMG seems to suggest the same:
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., 2 to 2, 2 to 1, 3 to 1, or even 4 or more to 1 basis according to the relative strengths.
Adjusting the XP for treasure ratio does not appear in Holmes or Moldvay, nor in Labyrinth Lord, Swords & Wizardry, or OSRIC.
It seems a little fiddly to use, but I'm still surprised I never noticed this before.