For those further interested in the opportunity cost question, the Library of Economics and Liberty is featuring this month L.S.E. Essays on Cost edited by James Buchanan and George F. Thirlby and including essays by Hayek, Coase and others.
This sentence from Buchanan’s preface caught my eye:
In any general theory of choice cost must be reckoned in a utility rather than in a commodity dimension.
Buchanan’s short book Cost and Choice is also available.