The mid range is supposed to be 300-3000 with an average of 1650 and the high range is supposed to be 400-4000 with an average of 2200.

I figured out the die size that would use the largest number of dice possible under 256 which would generate the same average value. Changing the mid range to 220d14 makes it 220-3080 with an average of 1650 and changing the high range to 220d19 makes it 220-4180 with an average of 2200.

Decreasing the number of dice and increasing the range does increase the variability in the treasure, but I waas able to determine that 999 times out of 1000 the mid range will actually generate something between 1454-1846 and the high range will generate something between 1932-2468. (I checked and 300d10 did have a tighter 999 out of 1000 range at 1487-1813. 400d10 broke the calculation tool I was using.)

I then looked at how often you would get a value outside the intended ranges (how often the mid range would generate 220-299 or 3001-3080 and how often the high range would generate 220-399 or 4001-4180), and it looks like if you do nothing but play these games for the rest of your life, your chance of seeing a single value outside the intended ranges is lower than your chance of hitting 17 consecutive Powerball lottery jackpots by buying exactly 17 tickets - 1 for each of the 17 Powereball lottery drawings.