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

Its a hard question to answer, and you need to be specific about which add-ons.  A DAC, after all, is an entire additional component. On the other hand, the chassis, jacks, power supply etc are far and away the most costly part of any component - so there is clearly economic of scope at work here.  But a good DAC is not cheap to add. A cheap DAC ...well . is :-)

IN small volumes everything costs if its done well.  But  lot fo the cost is a fixed investment in engineering and tooling.

As you see, its not a simple question.  Just buy what gives you what you want, and provides good value (sound/dollar). If the bets sound/dollar has gee-gaws, who cares?

Not only do the add-ons cost more money but the extras CAN cause sonic distractions as well. This is the premise behind stand-alone units. Many expensive preamps don't include tone controls, headphone amps, phono inputs, etc.

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.