If I remember right,,all BAR production before 1976 was Belgian.
After '76, the parts were made in Belgium and the rifles were assembled in Portugal.
All the Brownings made after '75 will have a 2 letter 'year code' in the serial number. Depending on wether they were Japanese, or European made and assembled,,the 2 letter year code is in a different place within the serial number,,but it is always present in the number.
The date codes use the following letters to represent numbers:
Z=1
Y=2
X=3
W=4
V=5
T=6
R=7
P=8
N=9
M=0
So a serial number of 23456RP789 would show mfg in 1978
With all that said,,the 'assembled in Portugal' BAR's have little collector interest. At least they didn't when I was casually interested in the Brownings. In a BAR, if it wasn't a Belgian made and assembled rifle, it was only a shooter no matter how nice it was.
..and they are nice shooters. The ones I've seen and shot functioned just as well as any of them.
Price??,,I don't know what they go for now. A search of the auctions with specifics to the Portugese assembled rifles should turn up some range of prices.