Yup, also, the incremental cost of space in a self-store unit is of the order of $1/month-ft^2, say $240/ft^2 capital cost at a 5% annual rate—and that is a true incremental cost. The more severe approximation is ignoring which items stack well and which don’t, and ignoring the additional costs of maintaining the items, keeping track of them and so on.
Yup, also, the incremental cost of space in a self-store unit is of the order of $1/month-ft^2, say $240/ft^2 capital cost at a 5% annual rate—and that is a true incremental cost. The more severe approximation is ignoring which items stack well and which don’t, and ignoring the additional costs of maintaining the items, keeping track of them and so on.