A good way to determine the value of a gun is to look at the averages on gunbroker.
The average value of a used silencer/suppressor? ZER0 (with some rare exceptions, such as a factory HK SD silencer)
There is only one listing on GB for a used silencer. It has zero bids.
Even for new silencers, there are zero bids. Look at this pic of the listed auctions for new silencers as of 5/9...
The reason for zero bids on new GB silencer auctions is the customary $50-$75 holding fee on top of the $200 tax stamp. The winning bidder must have the selling dealer ship the silencer to the winning bidders' holding dealer. This holding dealer will charge a holding fee. Holding dealers will customarily waive this holding fee for buyers who buy direct from the holding dealer.
That's right now. If not for 922(o), a used machine gun would also be worth a big fat zero.
But talk about the future prices for silencers, if ATF 29P becomes rule, then the oil can/solvent trap silencers might be worth more in the future (because of new marking rules, no one would be able to make a registered oil can silencer with the registered back end). That's just speculation, but if you can't make it anymore, people typically may want it at some point. I've spoken to some old time dealers who never foresaw the current prices of MGs ('M16 for $2500! What a rip off!', said one dealer in the late 80s).
Btw, I'm pretty sure that most scopes and most other accessories decline in value; they don't just float on par. There are exceptions of course, such as if you purchased a warehouse full of 22LR in the 90s and sat on it, a boatload of Colt snake guns in the 90s/2000s, or a ton of smokeless powder from the 90s. But I'm pretty sure any used Leupold is worth significantly less than the msrp.