Rick,
Interesting . When I use your original quantiles list.

data Quantiles;
input Amount Prob;
ECDF + Prob;
datalines;
25 0.15
50 0.16
100 0.21
250 0.25
;

And make sample size bigger and get much better result !
n = 1000000;

Analysis Variable : AMOUNT
Minimum Mean Median Maximum
2.00 288.70 93.80 29943.92