If you are buying it as part of the car you will pay the price from when you ordered, prices change all the time and this is what's happened (only exception being when the price comes down).
It would seem that people who choose not to buy FSD are very keen to tell other people the same. Yes it's expensive, it was under £5K when I bought mine 3 years ago and is just going to get more expensive. I bought this car for the tech, there was no way that would exclude the full set of features. I use it lots, lane change on the motorway is a huge benefit, makes a difference on a long journey, really makes the difference between getting home after working away or staying in some grotty hotel. I use summon quite a bit to move it around on the drive when I'm washing it.
Also FSD does not 'disappear' when you sell the car. It will stay with the car for future drivers unless you PX it to Tesla, sometimes they remove the option if it suits their cost model. That doesn't apply to selling it anywhere else. It is however an option, and like all cars your specific choice of options makes pretty little difference. It'll be interesting to see in a couple of years time.