Acho que estraguei um pouco o seu FK, mas pelo menos você tem o espírito;)
SQL Fiddle
Configuração do esquema MySQL 5.6 :
Consulta 1 :
SELECT
R.budgetid_fk,
SUM(R.quantity),
SUM(R.quantity * I.price * COALESCE(CC.amount,1)) as total,
B.budgetid,
B.budget_month
FROM tb_pro_request R
INNER JOIN tb_items I
ON R.itemid_fk = I.itemid
INNER JOIN tb_budgets B
ON R.budgetid_fk = B.budgetid
AND B.active = 'Y'
LEFT JOIN tb_currency_converters CC
ON CC.from_currencyid_fk = I.currencyid_fk
AND CC.to_currencyid_fk = B.currencyid_fk
WHERE
R.investmentid_fk = ''
AND (
R.approval_status = 'P'
OR R.approval_status = 'A'
)
AND DATE_FORMAT(B.budget_month,'%Y-%m') = '2018-03'
AND B.departmentid_fk = 'DP0003'
GROUP BY R.budgetid_fk
Resultados :
| budgetid_fk | SUM(R.quantity) | total | budgetid | budget_month |
|----------------|-----------------|-------------------|----------------|--------------|
| BU201803000001 | 7 | 575.2840143424692 | BU201803000001 | 2018-03-01 |
