Hello Twistedtree. I was at Paul Russell's place in Essex not too long ago. It was a pleasure visiting with the fellow who'd recently done some work on my 280SL. The people, not to mention the cars, there are amazing aren't they? I do recall reading on their website awhile back, though, that an owner will likely never recover the cost on a top quality restoration. The next owner will almost certainly be the beneficiary. Of course an heir might do extremely well if they inherit an iconic car. (not that a Pagoda is "iconic", maybe never will be ... but then take a look at 190SL value trends over the past few years).
Re your estimate that a top notch Pagoda restoration could easily cost $100K, I also heard something very similar to that just a few months ago, with $100K being minimal. Of course that assumes a solid car is delivered to the restorer to begin with. So, say one starts with a $50K car and puts $125K into the restoration process; then one may have the pleasure of ending up with a Pagoda worth around $100K, (but which cost $175K).
Based on the numbers I've seen it would appear that the top W113s, within the U.S. market, have over the past decade increased at a compound annual growth rate (CAGR) of a little over 7%, perhaps closer to 7.5%. If that level of appreciation holds going forward, (which may not be a terribly unreasonable assumption), then based on "the Rule of 72" one might anticipate the finest 280SLs to double in value every 10 years or so. So if the top notch cars are currently valued at say $100K then in 10 years they could be $200K, another 10 years $400K, and so on. Who can know(?), but there might be some logic to it. And maybe some comfort.
BTW, I've owned my 280SL since 1985 and do not plan to ever sell it. Bought it in Portland, OR. where there happened to be a shop doing a surprising number of 300SL restorations at the time. The guy even had one of the rare aluminum bodied Gullwings there for some work. I'm still kicking myself for not buying a gorgeously restored 300SL roadster, which could have been had for $75-80K then.