How much do I need to spend to get a preamp that sounds better than no preamp?

Good point George, I haven't tried a preamp made from light dependent resistors yet.  I did try Tortuga Audio's tube buffer as part of their "tour" last year.  It was pretty good and I suspect putting that buffer in-line with their LDR passive would probably sound good, although probably not as neutral as a SS buffer.
I have sometimes thought about constructing a version of NP's B1 buffer but modified to be balanced and to have a high quality attenuator and other high quality parts, but that is actually what my preamp consists of. I have not had a single instance where I used a passive and found it to be the sonic equal of my buffered preamp.

I tried a Freya. Not for.me.

I have sometimes thought about constructing a version of NP’s B1 buffer

Nelson did make first the B1 buffer for the Lightspeed Lightspeed so it could drive my customers low input impedance First Watts and Pass Labs.
It wasn’t known as the B1 then, that came latter when he did a diy around it for the masses.
Here is a simplified circuit of his for the Lightspeed, before it was called the B1.
https://ibb.co/choBZS

Cheers George