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

Hard to answer your question but that’s probably a good guess of if it were not including DAC.  The thing about preamps is they are becoming more like DACs with volume and inputs   Some DACs even have analog inputs.  
 

Spending more $$$ doesn't really add more bells and whistles.  There are a lot of budget preamps with tons of features and there are tons of expensive ones without many features.  I ended up getting a used McIntosh c47.  I love the McIntosh sound and it can do everything the newer and more expensive models can do.  The C53 has a few more inputs but the sound quality to my ears was identical, so I couldn't justify spending more then double the price.  

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