The Black Swan and Egret classes cost roughly half as much as a Tribal-class destroyer in this comparison. The Tribal needed the machinery and boilers for much greater speed. It also carried torpedoes, adding weapons, fittings, and support demands that the escort did not share. Procurement comparisons must account for the whole mission and propulsion plant rather than visible gun armament alone.
Notable Point · 29:59-31:09
Why a Black Swan escort cost less than a Tribal destroyer
The price gap between an escort sloop and a fast destroyer follows from machinery, speed, and torpedo requirements rather than guns alone.