Transaction Cost

can be hard cost (Credit Card fees, legal fees for contracts, etc.), also other fixed costs (Shipping And Handling)

time/Opportunity Cost - specifying project deliverables, thinking

Clay Shirky has argued that Micro-Payments will never be widespread, because even if the hard Transaction Cost-s are tiny, the mental cost (thinking "do I want to pay that nickel?") are annoying enough to reduce demand.

see also Ronald Coase's Theory Of The Firm