An excellent question!
There's a couple reasons for this. First, a full set of books will have more production cost than the single hardcover. This is mostly due to the nature of their being proportionally more things in the box! But of course, that's not the answer anybody wants to hear so let's talk about the good stuff.
Also, there are +EXTRAS that are exclusive to the tiers at $250 and above. Specifically the Limited Edition 10 Pin Cloisonné Set!
Finally, don't forget that you can always raise your pledge at a later date! You could even change it post-campaign through our pledge manager, so if you're unsure or want to wait until you see an +EXTRA that fits your fancy go ahead and back the lower tier until something good opens up :)