Bullgrit
Adventurer
You are thinking of the Basic D&D rules. The Advanced D&D rules say xp gain stops at the minimum for 2nd level. So the thief would stop at 1,251.(1) A 1st-level thief needs 2,501 XP to attain 3rd. Therefore, by my interpretation, 2,500 can be amassed before any more are wasted.
The 30/70% monster xp to treasure xp is not a written rule, but that seems to be the average break down judging from what came of a series of core AD&D1 adventure modules. And no one has argued against that ratio.(4) There is no rule decreeing that one must deal with 643 XP worth of monsters to acquire 1,500 GP worth of treasure. Nor is there any rule but whatever players enforce among themselves requiring equal shares of treasure.
Bullgrit