price, buying & selling
沽 centers on commercial exchange: the price of something, or the act of buying and selling. The two given meanings are two sides of the same transaction.
沽 combines 氵 (water) with 古, which likely contributes the sound. The connection to commerce may come from the idea of liquid goods being bought and sold, but the exact historical development is uncertain.
The left side is water 氵, and the right side is old 古. Imagine an old market by the water where prices are haggled and goods are bought and sold.
For コ, picture a coin (コイン) being exchanged at the old water market: coin -> コ, and the coin represents the price of buying and selling.