अतोऽर्धं किशोराणां च ॥ कZ_०२.३०.२४ ॥
ato'rdhaṃ kiśorāṇāṃ ca
From that standard valuation, half applies in the case of colts/young horses as well.
Young horses are not immediately deployable; the state must invest time and cost in training and maturation, so the purchase value is discounted to reflect delayed utility and risk.