Thinking about ways to revitalize downtowns and other urban commercial
areas, our foundation asks how profitable are parking meters. Do they
merely pay for themselves and their enforcement in fulfilling their
function of keeping parking places circulating or are they an actual
revenue stream for cities? We are Indiana specific and would pay a
bonus for research from there. However, any credible study would be
welcomed, especially those that do not rely on government bookkeeping.

How can any reputable study come up with any kind of financial value
of parking meters without relying on government bookeeping?
I assume that the government books have to be the initial source for
all revenues (meter collections, tickets ect) and expenses (meter
maids, parking enforcement, maintanace ect).

Here is a good list of costs and revenues from parking meeters listing
the annual net revenue per meter spot for a number of major
metropolitan cities...
http://www.city.vancouver.bc.ca/ctyclerk/cclerk/950711/ub1ii.htm