I share your enthusiasm (for paying more attention to the costs, benefits and impact pathways of transparency and open governance initiatives), and your scepticism (that the detailed, direct costings are the most interesting place to look).
The direct, administrative costs of open gov initiatives I would have thought will always be fairly small (compared to building a bridge, say) and will tend to come down with technology — but these direct costs are not necessarily the most important driver, or barrier to the impact of open governance initiatives.
It may be more helpful to think about indirect costs & losses on one hand and the gains on the other, from shifting any particular area of deliberation and decision making out of relative privacy into greater transparency. On whom do these costs and benefits fall, and are they enough to mobilise people either for or against the initiative? What are the incentives for the actors involved to act differently and how — officials, politicians, regulators, journalists, private contractors, competitors, citizens? As you highlight: politics.
And then I think the really important question is whether and how the costs/benefits of transparency/open governance initiatives can be non zero-sum — do they simply result in decision making being weighted in a way that transfers resources from one group to another? (which can be important, but is limited) or can they support a change in the effectiveness of organisations, and networks in solving problems and creating wealth? Perhaps this last set of questions is too difficult to answer up front(certainly it does not fit into the kind of accounting framework suggested by the costing initiative) — but when we look back on the invention of movable type, or the internet, or the secret ballot, or the joint-stock company or any other ‘social technology’ which changed the world understanding the detailed breakdown of initial set-up costs seem less significant than tracing the pathways of change that they set off.