Can be due by many reasons:

1) management fees
2) cheaper spread to lure customers (UOB )
3) internet banking for convenience (Public bank)
4) location of branch (the more the bank the easier to sell) - cimb, maybank

you mean personally? yeah, if you're trading a few tons at a time.

you calculate how much total cost you for each month
then divide the total to how much gram you bought to get even
you should sell only when the price is higher than the even price

eg:

on Jan, you bought 10gm, total = RM 1780
on Feb, you bought 10gm, total = RM 1680
on Mac, you bought 10 gm, total = RM 1580

so, total is RM5040 then RM 5040/30gm = RM168

Then, to get profit, you should only sell when bank bought at RM168/gm

Good idea on average cost VS current sell value.

IMHO, if the buying is long term periodic, then i think another, perhaps more relevant way, would be to add in time-value of money, ie. look at all the invested cost VS current value as a %pa compounded. To do this easily, checkout the function XIRR in Excel.

Just a thought 

Simple example
[attachmentid=2778375]
Thus, some example usage would be:
if / when the %pa compounded reaches your target profit, U sell some/all
OR
if U are value averaging down, when the loss %pa hits certain target, U put in an amount to buy "lelong" prices.
OR
???
Up to you to decide - no right/wrong . Just some possible trigger points and actions to bounce around.