I don't know if original exhausts are still available through Honda. I wouldn't be surprised. Then..., how many of you remember the price Honda then asked?
Actually CMSNL sells a set of 500/550 exhausts for the price it could be had decades ago, be it that price has been indexed for inflation. And David Silver asks even less. So?
BTW, I can strongly recommend the original 4-4 exhaust system. Only a 4 in 2 in 1 could in theory offer a bit of power increase. Such a system is not available. True, it's an investment, but with four that will make it a true Four, your bike will definitely have more value when you sell it. If I compare the expense with the money some are willing to spend on electronic gadgetry... Another advantage is that you have an excellent possibility for diagnosis. As a matter of fact you can finetune the mixture of each carb. At idle you can even listen to each cilinder. And there's the looks ofcourse.
Back then I've even sold my old set for $ 60,-. The new owner reasoned that the headers, that were still spotless, were already worth the price. With my first set I did some 75.000 kms. And the second set is still on (55.000kms). No short rides is the secret. For the rest a little polishing every now and then. Mine are still nice and shiny, but - I must confess - internally, some things are loose. The headers (to where the pipes become wide mufflers) can stay brand new forever by just cleaning them every now and then. Rust on that part is a shame and can easily be avoided. So, even when the mufflers have rotted, you can still have brand new headers left. Maybe not such a bad investment after all...