Por que você usa a cláusula "onde"?
mysql> select * from test;
+------------+------------+----------+
| request_id | unit_value | quantity |
+------------+------------+----------+
| 1 | 3 | 2 |
| 1 | 19 | 1 |
| 2 | 6.76 | 0.75 |
| 2 | 18 | 0.5 |
+------------+------------+----------+
4 rows in set (0.00 sec)
mysql> SELECT request_id, sum(unit_value * quantity) as x from test group by request_id;
+------------+--------------------+
| request_id | x |
+------------+--------------------+
| 1 | 25 |
| 2 | 14.070000171661377 |
+------------+--------------------+
2 rows in set (0.00 sec)
