Well, Physical Immunity with a stacked Catch (One thing, Easy to get around), is only -2 or -3 Refresh depending on whether everyone knows of your immunity or not. That's probably the way to go for full immunity to any one specific venom, or even poisons as a whole. I could see an additional +1 discount (for a total of -1 or -2) just because it's so specific (again, being immune to poison generally is the same price).
Alternately, you could come up with a Mortal Stunt to give a +2 bonus on rolls to resist the venom. Though, personally, I like the first option better.