What is the hardest part during the designing and manufacturing process of a Transformer?

I recently saw Retro Seaspray in a shop, and I always thought he was just a slight retool of the previous Seaspray figure, but he's way bigger! That made me think about the whole process of designing our favorite robot buddies.

The transition from Skywarp to Starscream is of course pretty easy for them. Give 'm a new color and you're done (and in Scream's case: a slightly new face). Then there is Windcharger/Tailgate, where it's just a case of a new color, a new head sculpt and some other small touches. But in the case of Seaspray... you'd think: of course, make the same bot with the same transformation mechanic, but make him slightly bigger! Easy! But is it that easy? Or is it basically as hard as designing an entire new toy?

I just would like to understand more of the whole process. For instance, 86 Prowl has the same head sculpt as the upcoming Bluestreak. People were saying that that's Hasbro taking the cheapest route, and that's probably true, but HOW much cheaper is that? Would the figures be even MORE expensive if they'd spend more time and money to details like that?

So that's what I was wondering: what takes the most time for them, and what costs them the most money? Is it the designing process (figuring out how to make a robot who can turn into a hovercraft)? Is it the sculpting? The manufacturing of the parts?