Yep lumens are additive (though your eyes perceive them logarithmically).

I don't know much about car headlights, but chatgpt says high beams are typically 25-45 watts, and assuming a generous 200lm/w that gives you 5000-9000 lm.

Roughly speaking, it's expensive because it's 50 lbs & tons of electrical components (that are much higher quality than $24 headlights).