Very good points, thank you! And yes, I'm considering these boxes as stretch goals because our price per deck is already pretty low. Based on what you've said, I think sticking with just the full-brick box may be a better approach, to reward those who invest at that level, and a single box for 12 decks adds much less $ per deck than a half-brick box does for 6 decks.
There's another option to consider, now that I think of it.
Say you have people who are interested in getting that cool brick box, but simply can't buy in at twelve decks. I've heard of some campaigns making the brick boxes available as an add-on purchase, where people simply pay for the box if they bought fewer than twelve decks but want that cool custom artwork.
It's even possible, for those backers, to ship their boxes flat and unformed - they have the option of making it up themselves, should they wish to do so, once it arrives in the mail, or leaving it flat for neat storage in a smaller space.
I would imagine for that to work, however, you'd need to make a really SPECIAL box, made with a wicked-cool stock and decorated with some extraordinary artwork on it that really enhances the appearance and makes people want it all the more - which of course makes it cost a wee bit more to make, but if you're getting people simply buying the empty boxes outright, it's not something you need to worry too hard about. But the higher the price point, the lower the odds you'll sell too many of them to less-than-a-brick backers, who may be hard pressed for cash to start with and disinclined to spend on an empty box if it costs as much as a deck or more. Keep the price point low while at the same time making it very cool-looking and you'll have interested parties.
It also allows you to sell more brick boxes than you might have otherwise - with luck, you'll increase the print run size to the point of lowering the per-box printing cost, making it a bonus all the way around.
...and of course, anyone buying in at the brick level gets one automatically for free.
Something that can never hurt your campaign - get a well-known artist to work on the brick boxes. Lotrek (go check out his campaigns...I'll wait...) has been known to print tuck boxes and brick boxes for other campaigns, and his work is in high demand. Yes, he has access to facilities where he can do his own printing, embossing, foil work, etc. and has a selection of special stocks. He's known for making several decks that are heavily adorned with if not exclusively made using stamped foil - some of his designs don't have a single drop of ink on them.
When my club, 52 Plus Joker, brought him on-board to make the annual club deck last year, not only did it boost membership and not only was it the most well-attended convention in the club's history (or so I was told), but his was the first deck in the entire series to completely sell out, and it did so on the first day. Jackson Robinson's club deck is also sold out, technically, but he made two versions - the silver, members-only version in the slip-cover box is long gone but the gold mass-market version is still available from Expert PCC.
But anyway, back to my point - well-known deck artist designing/making your brick box can add up to increased interest and sales. But there you run into the conundrum of well-known artist equals increased costs, so there's that to consider. You just have to really run the numbers and see if they add up right for your campaign.