More expensive = better?

Yes,  but there's a catch. 

Not all $8k integrateds sound the same. There are $8k integrateds disguised to look like $8k but sound like $2k so you've got to do some picking.

Assuming legit manufacturers, etc, a $150,000 preamp+amp(s) combo will always sound better than a $5000 integrated.

I would agree with this. Personally I don’t think you need to go up to $150k worth of preamp+power amp to beat a $5k integrated. A $50k pre power should show a $5k integrated a clean pair of heels. Having said that, to me, a quality integrated starts somewhere at $7k, new prices. The higher you go the better it sounds but one has to pick carefully, of course. Aspects such as preferences,  point of diminishing returns etc. will be applicable.

As mentioned earlier, some amps may be disguised to look $8k but sound like a $2k.