Решение

SELECT
    p.product_id,
    IFNULL(ROUND(SUM(p.price * u.units) / SUM(u.units), 2), 0) AS average_price
FROM Prices p
LEFT JOIN UnitsSold u
    ON p.product_id = u.product_id
   AND u.purchase_date BETWEEN p.start_date AND p.end_date
GROUP BY p.product_id;