The bonus is supposed to be equal to the number of miles earned for buying the ticket. So if, for example, you ticket to Manila costs HK$2000 in total then you will earn 250 miles on that purchase and get a bonus of another 250 if you buy it on the CX website.

Having said that, the price used for the bonus calulation seems to be slightly lower than the total fare paid. For example my most recent ticket costing $1974 incl all taxes earned a bonus of 239 AsiaMiles which would be $1912, so not sure where the other $62/7miles went. $1974 = $1650 + $120 departure tax + 2 * $102 fuel tax.