Support costs vary with respect to number of customers, and so are a true, and often overlooked, variable cost for shrinkwrap software companies. While many companies generate revenue with support, the basic level of support that customers reasonably expect (e.g., a web forum where people will make some effort to address bugs) is non-trivial to provide.
VC = TC-FC FC is always in the long run equal to zero, so VC = TC Marginal cost Which is what is seems like you are talking about is MC= d VC/d q where q equals quantity output. So in this case q would be the number of copies of the software produced or sold. Not necessarily the number of customers.
