It's not quite peanuts: when I had been replacing my AC system I was quoted the single speed system at $X, dual speed at $1.5X and variable speed at $2X. You need two variable speed motors: in the compressor (which is the most expensive part) and in the air handler (it's low power compared to compressor) and you also probably want a variable furnace, which is can be way more expensive than a fixed power furnace as you cannot just throttle your gas line to get less heat: that will cause condensate in the heat exchange and quick end of the furnace.