FSD Pricing on new and used vehicles

FSD's hit the point for me where I'd like to just permanently have it. AP doesn't seem to have an update in sight for it and FSD just handles straight line driving much better, even without it's other capabilities.

I have 3 options, buy it outright for 8k, buy it on the subscription for 100, or buy a used car with it already on there for an average 2500 additional(over other similar models).

Considering that our older cars are HW3, it seems like kind of a waste of time and money to choose the upgrade options. While I'm happy with HW3 I won't ever expect the same level of FSD performance from HW4.

Does trading out for another used older car seem stupid? I could also maybe get a dual motor this time. What did you do when you if or when you decided FSD was worth it?