cheapest way to fix heating and air conditioning?

"Fix" covers a lot. The cheap stuff you can do yourself first: change the filter, check the thermostat batteries and settings, make sure the breaker hasn't tripped, clear leaves and debris from round the outside unit, and check the condensate drain isn't blocked. That sorts a surprising number of no-cooling and no-heat problems. Beyond that - refrigerant, a capacitor, the compressor - you need a tech, and getting two quotes is the real money saver.