The relations between tables are not quite clear but are you looking for a correlated subquery? Something like
SELECT sa_id,
sa_date,
p_name,
cu_name,
sa_quantity,
sa_price,
sa_amount,
(SELECT Sum(pur.pur_quantity * pur.pur_price)
/ Sum(pur.pur_quantity)
FROM purchases pur
WHERE pur.pro_id = sales.pro_id) AS AVG_Price
FROM sales sales
INNER JOIN products ON products.p_id = sales.pro_id
INNER JOIN customers ON customers.cu_id = sales.cu_id