I don't like the hard cut off line of 75%. I would prefer a more graduated scale.
70% chance at ..9,000, 35% chance at 16,000
65% chance at 10,000, 30% chance at 17,000
60% chance at 11,000, 25% chance at 18,000
55% chance at 12,000, 20% chance at 19,000
50% chance at 13,000, 15% chance at 20,000
45% chance at 14,000, 10% chance at 21,000
40% chance at 15,000, ..5% chance at 22,000
This allows a few higher item possibilities, but doesn't get the market flooded with all the stuff just below the cut off line. A +4 belt should be more rare than a +1 sword. Extended the other way, there would be a 95% chance of finding something under 4,000 gp. That fits with the metropolis definition of nearly all minor magic is available.