[Not another shrubbery]

Hmm... Chris has good instincts, but it seems more natural to me to base the cost off the damage.

[Edit] I skimmed through the OP and completely missed the mention of Cycles *doi*. Yeah, that part shouldn't figure in IMO. The concept of Neutralize Poison seems to be stopping it before it damages you, so just stopping the next cycle should do the trick.