When you select a pair of boots in a gambling window, you can only choose among the 5 normal grade boots : Boots / Heavy boots / Chain boots / Light plated boots / Greaves.
Then, some roll occurs, to check if normal grade is generated, or exceptional, or elite grade. I assume you prefer normal grade (for the lower level and - more important maybe - strength requierement).
Chance to have exceptional / elite increases with your character level. And gambling cost also increases with char level : with a given amount of gold, you may gamble much more boots at char level 30 than at char level 99.
2 "breakpoints" exist concerning gambling cost : char level 25 (ability to equip Edge bow) and 62 (for Gheed's).
10fhr boots need affix level 5,
80%eg boots need 17,
40 x3 res boots need 25,
30frw boots need 37,
9 dext boots need 46.
Suppose you prefer 9 dext over 80eg : this is the item level required for each kind of boots to meet affix level 46.
Boots : iLvl 47
Heavy boots : iLvl 49
Chain boots : iLvl 52
Light plated boots : iLvl 56
Greaves : iLvl 59
Demonhide boots : iLvl 64
Sharkskin boots : iLvl 65
Mesh boots : iLvl 67
Battle boots : iLvl 70
War boots : iLvl 73
Wyrmhide boots : iLvl 73
Scarabshell boots : iLvl 73
Boneweave boots : iLvl 73
Mirrored boots : iLvl 81
Myrmidon greaves : iLvl 85
Simply add +5 to know what char level should be used if you try to get normal grade boots.
For exceptional or elite, you may increase your chances with level 99 but this would also significantly increase each gambling cost !
If I were you, I would use level 62 char (with Gheed) and I would gamble every boot I see, except Greaves.