It's pretty simple. AC units need to be cleaned and serviced on an regular basis. A simple cleaning of your condenser coil will go a long way to prolonging the life of your unit. You must clean your unit from the inside out. While your in there you should also take your shop vac and remove any leaf, or buds that may be in the unit. Last summer I went to a friends house and discovered his 10 year old unit had never been serviced and was done.
Cost to replace $4000/$5000.