I especially love the price drop!
By how much more should the price drop, before you would consider buying this puzzle?
I know I had my helicopter Skewb (60 x 60 x 60 mm) with about 160 parts printed for about 120 dollar including 19% Vat
While hollowing a puzzle out works very nicely, the real price drop comes from getting the density discount. Parts can be really close to eachother, as you have allready attempted. If that is not enough for the discount, you can actually try increasing the wall thickness to reach the density. If you do, it will likely be cheaper. You could even throw in some extra pieces, fun things, whatever. The 50% price drop is much more significant than a few cc extra to print, just make sure not to expand the "blank box".