I wanted to vote $10,000 for a 50% increase but i'm not going to b/c I don't see it as a realistic option.
We know form the Roadster 3.0 upgrade that the battery upgrade will not be inexpensive. so it is a better value to sell your old car and buy new.
What I plan to do is only when "the wheels have fallen off" of my current S85D, upgrade (edited to add: meaning I will purchase a new Model S with the) the largest D size available at that time. Since I do not need a 500 mile capacity battery, if my home charging issues become solved -- which is a big if -- I will consider a "smaller than max available" battery size as long as it has at least 200 mile range under normal driving circumstances. I think a 75D currently meets this requirement, but i'm not too familiar with the specs so
My daily commute is only 35 miles a day in total. I do not have a home charging solution (I live in a condo, and I don't have a dedicated parking space) and rely on public level 2 chargers and SuperChargers when I travel. It works ok for now as I charge once a week in summer and roughly twice a week in winter . however I do worry that b/c of the volume of Teslas set to end up on the road over the next few years my access to home charging (or in my case level 2 public chargers) will actually DECREASE b/c there are more Teslas vying for a limited amount of chargers, and I expect the volume of cards to outpace the level 2 charger build-out, or even, if I'm allowed to dream for a moment SuperChargers in cities for condo dwellers like me -- I just don't see it happening, at least in the USA -- any time soon. Which brings me back to having to splurge on the biggest, (tho not most ludicrous) battery available at the time.
I hope to keep my S85D > 8 years. what will be available at that time is anybody's guess. remember, with a new car, you do get the latest and greatest features AND a reset on the warrantee of those pesky door handles, that pesky 12 Volt battery, the $5000 main computer screen that likes to fail on some people. as long as Tesla has a death-grip on parts, buying new is the way to go for a myriad of reasons far beyond getting the biggest, baddest battery.