Peter Suber: The OA Journal of Computational Geometry released its financial statements for 2010, 2011, and 2012 (YTD), „to help others understand…the real cost of running an open access journal.“
JoCG pays $10/year, for its domain name, and nothing else for anything.
Not all OA journals run on a shoestring this thin, but here’s one that does and that can prove it
For some others, see the March 2010 article in which Brian Edgar and John Willinsky report that 29% of 998 journals using Open Journal Systems claimed zero expenses, 20% claimed expenses between $1-$1,000, and 31% claimed expenses between $1,001-$10k. 44% operated on zero revenue, 16% on revenue between $1-$1,000, and 24% on revenue between $1,001-$10k. (See Table 15.)