A flatpack for a single sub is $1660, so double that is $3,300. This could be a starting point for determining what to charge. Anything less than that and you're practically giving them away. Of course, none of that includes labor and materials costs which are substantial, but for DIY, we don't expect to recoup much of that. The buyer is likely someone who doesn't already own a pair of these subs and they'll have to decide between DIYing or buying them used. If I were in the market, I'd gladly spend $3,500 or a bit more to buy them, but they would have to be a local sale. If you're considering building crates and putting them on a pallet to ship them, that's a huge additional cost. However, if you're only going to offer local pick-up, then you're severely limiting your audience and they should be priced accordingly (i.e., insanely low).
These are the holy grail for subwoofers, so why are you selling them?