How to value a used CD collection?


I have a pal that digitized all his CD's and is looking to sell them.  Approximately 500 high quality jazz CD's.   I expect he will be looking for a price for all of them.  I'd be keen to have them, but have no idea what that sort of collection is worth.  Maybe $5 a CD?, but he has lots and lots of stunning recordings that cost $30 or more.  Any insights would be appreciated.  Maybe I should just asked to match his best offer and see what happens.  

Any insight would be appreciated.  Thank you, Mark  
whitestix
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Of course but truly it is no different from what we all used to do back in the 70,s when we recorded our records onto cassettes.
Ostensibly to play in the car of course, nudge, nudge, wink, wink.
Never would have dreamt of keeping the cassette copy if I sold the actual hard vinyl copy...….
But on a lighter note that was really funny until I discovered upon death my parents had been renting the cute little princess rotary phone in the master.... for wait for it....30+ years...
@viridian