Any Belgian A-5 has value. A round knob 12 ga. plain barrel that hasn't been messed with would bring about $350 in the condition as shown in the picture. (It looks like the stock has been sanded). If the metal finish is good and is original, closer to $450. A round knob in mint condition would bring about $1000 from a collector. Condition is everything to a collector. An old Blue Book of Gun Values shows a value of $250 for 60 percent condition and $295 for 70 percent, but they're worth more than that now. The Belgian guns were made until 1976 and then made in Japan. they were discontinued in '97 or '98.
As a shooter, they are fine guns. They used to give problems with the old paper body shells if they got damp, but are very dependable with the plastic body shells if the friction collars are set right. I like the drop of the stock and the natural sight line from the flat face of the back of the receiver. Seeing the barrel move when you shoot takes a little getting use to, but you can. I also like the sound when they cycle. No stampings, no aluminum, and no plastic. Just machined steel.
If you enjoy the gun, you may be able to find a Japanese removeable choke vent rib barrel that will work with it. You can shoot steel shot in them. If you do, be sure to test the fit on your receiver before you buy it. (Some of the old guns show signs of some hand fitting). Also, if it's a 2 3/4" chamber gun, be sure not to try to put a 3" chamber barrel on it. The 3" magnum guns have heavier receivers, a different friction collar setup, and probably some other critical differences. I don't know if a 3" barrel will even fit at all, but I would hope that Browning did something to prevent it.
If the metal is nice, you can also find new and used buttstocks and forends to spiff the old warrior up.
Hope I helped, Tom