I think how much you can charge for them will depend on what they're made of. Are you still planning to do vinyl? The cardboard stashes were something like $15, right? I think keeping them in the $15-$20 range would work. And you can probably get away with them being closer to $20 by playing their durability over the old school cardboard ones. (Yay waterproofness! Also, vinyl 'mids will probably stand up to being dissassembled regularly to lie flat for storage.) Possibly a bit more to take the giant die into account. Maybe offer a slightly lower price for the non-colored ones? Of course all that is going to depend on your profit margin/overhead. But in terms of price tag, if I saw a giant stash in a store, I wouldn't think $20 would be an unreasonable sticker price.
Currently being lazy and not painting his last 2 stashes of giant 'mids so he can play Giant Volcano