There is a market for A-5 as they have a strong fan base of collectors, my father being one of them. His first gun was a Remington Model 11, built on Browning's patent of course. He now owns both 20 ga and 16 ga models.
From what I've observed, a 12 gage is the least desirable to collectors and probably not coincidentally the most common. The Belgian-made factor is not the price elevator many people think it is, the problem being that regardless of where they were made, they made a lot of them. Finding a Belgian made one isn't exactly a needle in a haystack proposition, especially now in the days of the internet, so I wouldn't expect a huge price price increase because of it.
It depends on what you think of as "valuable" and obviously everyone's opinion of what 90% condition looks like will vary. In general, I've found people tend to over-estimate condition and when buying on the internet the sale price has come to reflect that in the sense that buyers assume conditionis overestimated and bid accordingly. Local markets also vary.
My guess would be $550-650 for a private sale. An exceptional example might fetch $700 or more on a good day and with luck in having the right buyer see it. You have to keep in mind, while online sites may have a lot of guns listed for silly-money prices, that doesn't mean that's what the guns are worth. Guns are worth what they actually sell for, not what they are listed for.
Grouse