Apologies for not including the link there (thanks Milli).
I don't want a precedent to be set for paying for individual missions either. However, I think a large and well constructed campaign represents a lot of work that I would be willing to shell out a few bucks for. I don't know, what do others think?
Also as I only really play single player, it is the campaigns that I particularly enjoy, so maybe I am weird. Are there other freely available additional campaigns for a-10c?