A design that's for sure not going to change during the project nor needing rebalancing stuff saves time but not all that much. Budget will still have been rather high, close to any other made from scratch game...
By "a design" you mean every level, the enemy encounters, the enemies themselves and their behaviors, bosses and their behaviors, the combat mechanics and how they should work, the leveling systems, the weapons and armor that exist and what their stats are, any story elements, any missions and what they are. And all of those things had to be conceptualized and then iterated over at various stages, and a whole lot of things scrapped. Basically everything about the "game" is now something to copy and make an amazing looking version of.. which is by no means super cheap, but it's a totally different kind of project with definitely a smaller budget than all of the above. And sure they'd have iterated over some re-designs too.. but when the entire goal is a faithful recreation a huge portion of the overall process is just crossed out. Reducing all of that to what you just said like "rebalancing stuff" is just insanely reductive; and you seem to just be completely forgetting that it's not just a design that doesn't change... it's a design that this project didn't have to pay for, just copy.
I totally get that it's a lot more than a remaster though. Potentially one of the more expensive remakes ever.. not that there's that many lol To go to the extent they did is impressive.
A brand new game trying to break new ground with a new IP is also more expensive than a sequel.. they all cost the same price generally either way and I'm not complaining. We have games with budgets all over the map all releasing for what we consider "full price" and there's nothing wrong with any of it.
I'm happily paying $70 because it's an awesome product, regardless of it's relative cost to anything else.