Make it like the custom Peavey guitars. You can choose features like the core, springs, stickers, etc to determine a price, plus what cube. Designed online with either your picture files for each side, or choose colors they supply. Choose plastic color- each a different price for how much is left, rarity, etc. Than a total price without shipping at the end. So like this:
Good core with good springs, etc- $20.00
Blue Glow-in-dark plastic- $10.00
Default stickers- $0.00
$30.00 for a custom 3x3, maybe more maybe less cause i'm not good at determining Cube prices.