Go to Lowes and buy the locksets for whatever you need (deadbolts, doorknob locks, whatever). They sell packs that have both the deadbolt and the doorknob for the same key. They are under 20 bucks for a set. All you need is a screwdriver and 10 minutes to change them yourself. It's not rocket science and it will save you 50-100 bucks for someone to come out and do it for you.
Also, you can rotate locks between apartments over the years when people move out (just make sure you have 2-3 spares to switch it up a little).