I don't buy or sell W113's. I inherited mine, albeit as a "basket case" that was more suited for the crusher than a restoration...it's also a family heirloom, so I'm not likely to sell it unless something untold happens. Thus, my interest in pricing is mere curiosity.
You should know that Mr. Dearborn makes a market in trading W113's, among others. Thus, full disclosure is in order: he has an interest in seeing all the prices higher. Not a criticism, but the facts.
Taking one step further, with some minor exception (such as for a full engine rebuild because of some engine parts pricing) essentially, the cost to "restore" a 230SL is about the same as for a 280SL--many parts are identical or at least identically priced; plating a bumper, etc. about the same. But, "until now" if one is to believe some of the recent commentary, the later cars, both 250SL and 280SL have always commanded a higher price. One would always see a better return on "investing" in a later model than an earlier one--whether that investment is in the entire car or in restoring all or part of it.
Thus, I'm not convinced of his pricing commentary particularly since his standing on the advisory board of SL Market Place newsletter, and their price listing, seems contrary to his opinion here. Not that I don't believe there may be truth to it, nor do I doubt his integrity--just need to see it to believe it. If anything I suspect that the differential in pricing may close--with earlier cars coming up and the difference between a 230SL and 280SL narrowing.