footbridge, toll, levy, monopoly
榷 centers on a controlled crossing point: a footbridge where a toll or levy is collected, which extends to the idea of monopoly or exclusive control over passage or trade.
榷 combines 木 (tree/wood) with 隺, which likely contributes the sound. The character originally referred to a wooden footbridge and later extended to tolls and monopolies.
A wooden 木 structure with a high, bird-like 隺 shape: imagine a narrow wooden footbridge guarded by a toll collector who has a monopoly on the crossing.
For カク, picture a wooden toll gate that makes a loud 'kak' sound as it closes: kak -> カク, and you must pay the levy to cross the footbridge.