... the cost of adding a feature isn't just the time it takes to code it. The cost also includes the addition of an obstacle to future expansion. ... The trick is to pick the features that don't fight each other.
Poor management can increase software costs more rapidly than any other factor.
Cringley's Second Law: Ease of use with equivalent performance varies with the square root of the cost of development. That means that to design a computer that's ten times easier to use would cost 100 times as much.
60% of product cost comes after initial shipment.