The upgrade from 85kw to 90kw is $3000, or $600/kwh. The cost of a new 90kw pack is $25,000, or $277/kwh. Let's just use a round number and estimate low and say this new frunk pack would be 45kwh @ $300/kwh = $13,500. This assumes that the R&D for this completely new pack is free, it drops right in with no issues, and somehow just works. Of course that is impossible, so then you have to figure out:
- What's the R&D cost for this new pack?
- What is the cost of the other modifications to the Model S to make it work?
- What is the cost of re-crash-testing the Model S?
- Etc.
Let's just pretend you could accomplish all of the above including the base $13,500 cost of the pack for $20,000 even. We know that 50% more capacity won't give you 50% more range. Let's say it gave you 35% more range, or about 90 miles. Would you pay $20k for 90 miles? For some that might be an easy yes, for others, less so. Some people are paying $3k for ~15 miles right now with the 90kwh upgrade, so there's clearly a market for it (I personally don't understand paying $600/kwh but to each their own).
But then we have to get back to reality. Would you pay $20k for 90 miles, worse handling, a significantly reduced safety rating, and no cargo space in the front? Particularly for the safety reason, I doubt Tesla would even offer you the choice. But I'd think most people would say no anyway. I certainly would not want ~350 pounds of batteries to try to come smashing through my dash in an accident.