I could see something being applied as an incremental thing, but I somewhat agree. Keep the cap at 500k, and maybe do something based around player salary…
0-5k, start auction at 2x player salary
5001-10k, start auction at 3x player salary
10001-15k, start auction at 5x player salary
15001-25k, start auction at 7.5x player salary
25001-50k, start auction at 10x player salary
50001-99999, start auction at 500k
100k+, start auction at salary
Something like that, perhaps?