In room 19 you can drop ice marble, yeti fur+, yeti fur, piece of ice and piece of ice+, rarely clam and lv 75 weapons.
In room 32 and 35 you can drop ice marble, lv 75 weapons, lv 55 shoes, lv 60 helmets, lv 61 shields and clams.
In HL2 you don't drop ice marbles or any upgrade materials because there are no ice monsters.
I'd farm only in HL1. The more options you have in the drop list, the better are chances to maximize your sales.
what's the minimum lvl to have drop?