I want to have this citation expunged off my record, need some advice?

i have a question for any attorneys or people who have had an expungement. i recieved a citation for misd. larsony i wasnt the thief but the person whom i was with was lets say wrong place wrong time and some very overzelous cops. well anyway i went to court on the court date well over a year ago and the same day i plead not guilty, given a public defender and he later that day called and said the district attorney dismissed the case. well ive never had a record before this or after i was never arrested and ive never even gotten stopped by a cop or recieved a traffic ticket. im a good person i dont understand why if it was dismissed why it wasnt taken off i have always been in law enforcement im tryin to get back into it but fear this will effect it. how much does an expungement cost?