price, business, selling
估 centers on commercial exchange: setting a price, doing business, or selling goods. The idea of valuation connects the given meanings.
估 combines 亻 (person) with 古, which likely contributes the sound and may relate to old or established things. The exact historical development is uncertain, but the character is associated with commerce and valuation.
A person 亻 beside 古 (old) suggests an experienced merchant setting prices and selling goods in a long-established business.
For コ, imagine a merchant calling out 'Co!' as a short, sharp sound to announce a price: Co -> コ, and the person 亻 beside the old 古 shop is selling.