Is this "license-only" money?
If it isn't you can follow
@Anupam 's solution, but give me the money instead (Just Kidding on that Part
). With that being said, you can spend your hard-earned money not on a license for an antivirus but something you actually want. I suggest Avast! Free for the UI and feature kit, or Qihoo 360 for the lightness and detection (It often ranks high in tests and uses quad-engines or more).
If it is license-money, you should get Emsisoft instead of Bitdefender. If you want Bitdefender, there are some 3/6 month giveaways posted on MT every month or so. I would actually recommend ESET if you want to sacrifice a tiny bit of detection rate for stellar (small) performance impact.