it's an ok idea but dollars spent doesnt equate to the value gained, and will vary between locations. What we'll end up with Wikimanias in 3rd world countries because ts cheaper not because of potential impact. That is something we already see.
How do you measure impact when its about sharing of knowledge, WMF can ask for reports but a person who writes a good report may provide lower impact than a person who gets a one off opportunity and then builds and runs 100's of outreach projects over time but cant write a "good report". We fail to value the relationships, and personal impact for measures that can be put into easy numbers immediately. We do need to measure outcomes some how, all too often the numbers become centre of the universe not the sharing of knowledge. We would do better to stretch the limits rather than to shrink the dollars per unit, no where should be impossible, to small, or too far.