How much $$ do bells and whistles add to preamp?

I agree , some of the best sounding preamps are minimalist designs .   I prefer separate everything as it opens upgrade paths and helps put off obsolescence 

I had a Belles Aria integrated amp and loved it!  
 

@rustler , why not go integrated?  Do you already have an amplifier?

I’ve heard Audible Illusions preamps are nice.  If going separates and just starting out, I’f look into tube preamps, ideally with a sub out as well

If a preamp with an outstanding volume control as well as the ability to adjust the amount of gain on each individual input is important, consider the Coda 07X. It’s volume control is the best in my limited experience.

Integrated might be a sound option (hehe)

Seriously, I stumbled across Odyssey Audio a couple years ago ... thank the Lord!

Give them a look!

Have a look at a canary c803 there is a nos one on eBay at the moment it is a four chassis design so true channel preamplifier. Each channel has its own power supply and each channel has its own volume control.  The volume control is a stepped resistor ladder. Is is at 7500 USD. This is at the very top end in my opinion. So that would be what I would like if I wanted tubes. 

Solid State my favorite is Ayre k1xe preamplifiers. Same thing it has  a stepped resistor ladder for a volume control. 

Now to your question minimalist design s are generally more money than ones with a bunch of extra junk on them. That being said there are some cheap minimalist design preamplifier s that only offer one feature which is low sound quality. 

Regards