To tell the truth, I don't think the prices of the SOHC4s have gone up at all, relative to the prices of other things.
For example, when I bought my CB550 back in the mid-nineties, I paid $650.00. At the time, my rent was about $700 for my studio apartment on the upper east side of Manhattan.
Right now, I figure I could sell my CB550 in MUCH BETTER CONDITION (near perfect, really) than when I bought it, with many upgrades, including Koni shocks, electronic ignition, LEDs, much improved cosmetics, stainless steel brake lines, billet aluminum turn signals, and various other upgrades and improvements for maybe $2500 at best. The same apartment in Manhattan easily rents for $2500 right now.
In all those years I spent nearly $5000 in maintenance, parts, repairs, upgrades, etc.
So, relative to the cost of living, I think that SOHC4s have not really appreciated at all, with the possible exception of the highly sought after sand-cast model.
In fact, I see SOHC4 CB750s regularly on craigslist for well under $1000 in "when I parked it 15 years ago it ran fine -- needs carbs cleaned, new battery" condition all day long.
However, I don't think SOHC4s have depreciated as much as other bikes have, either. So, maybe it wasn't the worst investment. If I had bought a Kawasaki 400 ltd back then, for instance, i'd have to pay someone to drag it away today. Plus, I got to ride my relatively cool CB550 for all that time, so I'm happy.