Power Grid is a push and pull game. In some rounds, you grab a new station, and find yourself short on money for resources and houses. This may also mean that the next round, you will face lousy prices for both of these, too. In other rounds, you may have received a nice cash flow from those new power stations, and can indulge into the new houses as you don't need to spend another 20-30 coins on a new station for a couple of rounds, and can concentrate on the network or getting more resources.
Sometimes, it may be a good idea to stash some money and be the first one to break into Phase II and build the best second houses with no connection fees. This means you would need to have some 50 free coins to do that, so you may have to plan for it by building the power station capacity beyond 7, and bunking extra coal to fire up as many houses as you could -- basically, only buy houses on that turn.