My BMW Dealer did mine for 39.99 - it's an offer they have on at the moment. Check your dealer of your make of car - they may have a spring offer on that's very similar.
First - change the air filter - the one for the ventilation system not the engine - second go to your dealer and purchase an aerosol can of air con cleaner - usually �10 - �20 and use as per instructions -
generally you put the air con on max and recirculate and place the aerosol inside the car and press the button on the aerosol which empties the entire contents over a short period of time and gets into the air con sytem cleaning it - so i've been told - never had the need to do it yet.
change the pollen filter as woodchopper says. sometimes if air con doesnt get used a lot u get a damp cats ***** smell. u can buy an aerosol of odour kill that sprays down the vents usually that sorts it.