You will get BPs depending on how many attacker's troops were killed in the attack and on how many troops you had in the city when the attack hit ;-)
For instance, you had a lot of troops (and nobody else had troops there) in the city and a lot of troops were killed from the attacker - you will get a lot of BPs.
You had very few troops there (and some other players had a lot of troops there) and lots of troops were killed from the attacker - you won't get a lot of BPs.
