I'd say you should charge more for less and less for more.
Typically economies of scale work the more you buy the cheaper it is. Obviously writing is alittle weird as it is a lot more work.
The issue is if it's cheaper to buy 2 5 pa ks then 1 10 pack you'll never get someone buy a 10 pack when they can just order 5 packs 1 at a time
Either way best of luck; and to buyers his posts get comments I'm quite happy.