forehead, tablet, plaque, framed picture, sum, amount, volume
額 centers on a flat, prominent surface: the forehead as a facial landmark, and by extension a flat inscribed tablet, plaque, or framed picture. The sum/amount sense comes from the idea of a fixed figure displayed on a surface.
額 combines 客 (guest, but here likely used phonetically) with 頁 (head), originally referring to the forehead. The meaning later extended to flat inscribed objects and amounts.
A guest (客) arrives and you notice their head (頁) has a prominent forehead. That flat surface is like a plaque where you can read the sum of their character.
For ガク, imagine a guest (客) with a huge forehead that looks like a gong (ガク) when you tap it. The gong sound cues ガク and ties to the forehead meaning.
forehead; brow
(picture) frame; framed picture
amount of money; sum
large sum (of money); large amount
half the amount (of money); half price; half fare
total amount (of money); total sum; sum total
total; full amount; sum
monthly sum; monthly amount
large (amount of money); huge; hefty; expensive
fixed amount (of money); fixed sum; flat sum
huge sum (esp. of money); enormous sum; massive amount
reduction (esp. in price or cost); diminution; abatement
balance; difference (in price, cost); margin
increase (in an amount of money)
face value; par
small sum (of money); small amount
the same amount
(picture) frame
amount of tax
valuation; amount