Presumably you wouldn’t work out how much it costs you to produce a member (or $67,000 in donations that wouldn’t have been made without you, which it sounds like is your current estimate per member) by dividing the number of new members per year by your budget? That doesn’t speak to whether these members pledged or gave due to particular activities you spent money on. Where’d I find your latest calculation of this?
The $67,000 is an average over all our members, given how much people estimate they would have given over their lives were it not for us, and also whether they think we influenced where they donated. So, it includes the fact that many people who are members of GWWC were influenced to donate a significant proportion of their income to effective charities for other reasons. But, as you say, it’s more accurate to look at cost per marginal member for particular activities, rather than an average like this.
You can always find our most up to date calculations to estimate our effectiveness (in a number of different ways) at givingwhatwecan.org/impact.