No, AFAIK there isn't. People have just noticed that elemental attacks are not causing them much of a problem after they get their resistances up near 100, or so. You never become immune to elemental damage, for practical purposes you don't have to worry much about it.
IIRC there use to be a brief description of combat mechanics on the DivineDivinity.com site, but it no longer exists and I don't think it went into any actual calculations, anyway. I got the formula there for the experience required to reach a certain level (for the post on the Wisdom skill), but I think the rest was more of a general overview.
I suppose you could always use
iZakaroN's SaveEditor (or from
Kiya's site, with description) to give yourself a large number of hitpoints, then remove your equipment, make a test save and let the same creature beat on you with different resistances. There will be some range of damage in any elemental attack, but changing resistances should effect the average damage of a number of hits. Poison damage (ie after a zombie hit) might be more consistent (so you would need less values to determine an accurate average), but with less damage done it might be harder to see an effect from resistance. I'm not sure if dropped poison flasks will damage your character, but Buad's tea will.