There is also the standard 'supply and demand' rules in play. If you are looking for a C8, they seem to depreciate the most as there are about a bajillion available out there. I am a fan of the Intes scopes and they are selling for just a little below their MSRP as they are discontinued, have a good reputation and there were relatively few produced. The economy comes into play also... There are many factors involved.
When I make a scope purchase, resale value is never a consideration. If I was planning on spending a huge sum of money on a higher end RC, I'd be a little more committed to making the right decision upfront it terms of quality and specifications, and plan on keeping the scope for more than a year.
If you are concerned about depreciation, you could always purchase off the used market and sell it for what you paid for it.