For some reason it's surprisingly hard to figure out what these are currently going for.

I stumbled across a very clean/low round count one on a trade a year or so back, dropped in a Killer Innovations Velocity trigger shoe, SLR Rifleworks magwell, and 2x SLR mag extension baseplates for it (roughly $340 in parts).  Problem is it's been a safe queen ever since I bought it and I have other stuff I'd rather have taking up space in the safe, plus I'm primarily stocked up with 9/45 and edc a 9 so this G20 is destined to never get used as long as I own it.  Got a friend of a friend interested in the G20 but not sure what pricetag to put on it, it would have to sell w/ the upgrades because they're G20 Gen3 specific and useless to me elsewhere.

Used Glocks with upgrades do not really retain the value of the upgrade parts.  

For many that actually decreases the value.
I typically avoid used upgraded Glocks.  

That said, I have seen several used G20s in the last several months for $399.

I would put it back to stock and see if I could get $450 for it knowing I may need to negotiate down to $400.  
Then sell the parts separately.

I would have a hard time selling the upgrades outside of EE and that comes with its own headaches.  No one I know offline has a G20 Gen 4, in fact I don't even really know any other Glock guys.

Through trade value, I have $400 in to it plus the roughly $340 spent on mods, so call it $740.  If he really wants a G20 Gen4 and likes the mods, I guess I'll just tell him $600 and see what happens.  Worst case, it continues sitting in the safe until it gets used in a trade or eventually passed down to one of my boys with everything else I may or may not have impulse bought over the years.