To me, and this is just me speaking, internal upgrades don't add any value to the gun.
1: No longer stock/virgin.
2: Will break sooner.
3: Nothing fubars a gun faster then upgrades done by someone who didn't know what they were doing.
Buying a gun with upgrades is a risk.
I have seen many guns for sale like the following:
and thats it. That thing will break down the road guaranteed.
I had one case where I bought an upgraded G36, it worked like shit. The guy said it worked fine. Really? No.
I have bought 6 different aegs over the past year and all of them, excluding the G36, were stock and worked great.
As to the value of the guns. Supply and demand. If all the guns were priced at 50% of original value, the for sale section would be emtpy.