Steve,
Your projections for rounds at northern courses are good, depending on which market you're in. Your southern projections can be cut back by quite a bit, unless you're talking about California. Instead of double the rounds of northern courses, southern courses host around 20-30% more rounds.
That said, rounds have only a moderate influence on the cost of maintenance. There is a basic level of maintenance somewhere in the $250 - 300,000 range which cuts the grass on a regular basis and keeps the course in an acceptable muni range. As you step up the scale of quality to daily fee, high end daily fee, private, premium daily fee, and premium private, customer expecations demand additional levels of "finish" on the maintenance side. Each has a cost, in the form of additional equipment and supplies, but labor usually is the biggest line item to expand.
Some design features, such as bunker design, have additional cost factors as well, but again, the reason that private clubs spend $1 million is the level of detail demanded by their members.