The original text suggested 5,000 gp per current hit die, which seems like it would still work rather well. Thus, 30,000 to reach 7 HD, 35,000 more to reach 8 HD, and so forth.
Not so sure about it myself, it seems an awful lot compared to the "
Treasure Values per Encounter Table". Bear in mind AD&D creatures often had much more gp value in treasure than 3E ones, since the rules did not favour the purchase of magical items.
Look at how the progression goes Treasure EL-wise.
To reach 7HD requires 30000 gp = ~EL 17 Treasure. [36000gp]
To reach 8HD requires 35000 gp = ~EL 17 Treasure. [36000gp]
To reach 9HD requires 40000 gp = ~EL 18 Treasure. [47000gp]
To reach 10HD requires 45000 gp = ~EL 18 Treasure. [47000gp]
To reach 11HD requires 50000 gp = ~EL 19 Treasure. [61000gp]
To reach 12HD requires 55000 gp = ~EL 19 Treasure. [61000gp]
To reach 13HD requires 60000 gp = ~EL 19 Treasure. [61000gp]
I'd rather go for something like treasure EL = current HD+3, e.g.:
To reach 7HD requires EL 9 Treasure. [4500gp]
To reach 8HD requires EL 10 Treasure. [5800gp]
To reach 9HD requires EL 11 Treasure. [7500gp]
To reach 10HD requires EL 12 Treasure. [9800gp]
To reach 11HD requires EL 13 Treasure. [13000gp]
To reach 12HD requires EL 14 Treasure. [17000gp]
To reach 13HD requires EL 15 Treasure. [22000gp]