I don't see semi's being any different than bolts in that they all depreciate, except that high end customs tend to do so less, even though there are fewer buyers.
If you own a custom from a well known builder, that was done "right"...has a low round count and you've already done the load development- you may be able to sell it for MORE than you paid for it, if you find the right buyer that likes what you have and doesn't want to wait six months to get it...
I would think the same would hold for high-end semi's.
Then, there's another line of thought altogether.
Buy a $500 AR on sale. How much can you possible lose on it at resale?? You'll always be able to sell it for pretty close to what you paid for it in good condition.
Can't say the same for a $1000 AR.