That depends... if it's easy to reach and replace, then you can buy the cheap ones for a buck or less (try a dollar store or someplace similar that carries light bulbs)
If you have to climb a ladder to 25 up on the corner of a house and stretch out as far as you can to reach the bulb (like I do), then pay the couple of extra bucks (3 or 4 total) for a better bulb.