iLvl = cLvl-5 to cLvl+4
say your char level is 90, your gamble iLvls will be from 85 to 94
amulets and rings prices are static, other items i think prices go up as your cLvl goes up
so there's probably a price formula like: price = base value + qLvl*cLvl*value
i don't think vendor, player count, mf, or difficulty make any difference
more info here:
http://classic.battle.net/diablo2exp/basics/gambling.shtmlbut keep in mind in classic you won't get exceptional/elite items from gamble so ignore that part
here are some best levels for good items to gamble
ring - clvl 73+
boneshield - clvl 79+
bonehelm - clvl 84+
amulet - clvl 95+
if curious about any other item types i can check affix levels to come up with a level for those too lmk