ガク, ひたい, ぬか
forehead, tablet, plaque, framed picture, sum, amount, volume
JLPT2 G5 S18 F407
客# guest/customer + 頁 head (→ group of people) → *notice board* to attract guests/customers → *amount* (← fee displayed on a notice board). *Forehead* is a borrowed meaning, via a character combining 頁 and 各 rather than 客.
[Full Kanji Etymology List]