Your furnace's flame sensor is essentially a safety mechanism. As a thin metallic rod in front of the flame inside the unit, it's sole purpose is to confirm that your gas valves only open when a flame actually exists to burn that gas. When the flame sensor stop working, on the other hand, gas leaks can occur. To repair your furnace's flame sensor, expect to spend between $80 and $250. Even a full replacement of this part typically does not go above that range.
High-efficiency condensing furnaces (90% AFUE and above) are a bit more complex than conventional furnaces. The main differences between a conventional and condensing furnace are the heat exchanger technology used to extract heat from the combustion process and the method used to exhaust the combustion gases. In these ways, the furnaces are very different. The condensing furnace does not have a significantly more efficient combustion process than does a conventional furnace; both use gas burners with electronic ignition. The difference lies in that the condensing furnace has a more efficient heat extraction process after combustion.