When is zero a good approximation

I was commenting on Andrew Gelman's blog when a nice commentator pointed something out that I usually don't think much about: pharmacoepidemiology outcomes include both cost and efficacy.

Now, a lot of my work has been on older drugs (aspirin, warfarin, beta blockers are my three most commonly studied drugs) so I have tended to assume that cost was essentially zero. A years supply of aspirin for $10.00 is an attainable goal and so I have assumed that we can neglect the cost of therapy.

But does that make sense if we are talking a targeted chemotherapy? In such a case, we might have to weight not just the burden of additional adverse events but the cost of the medication itself.

It's becoming appalling clear to me that I don't have a good intuition of how to model this well. Making everything a cost and assuming a price on years of life lost is one approach but the complexity of pricing involved (and the tendency for relative costs to change over time) worried me about external validity.

I know what I will be thinking about this weekend!