SELECT 
  cscart_product_prices.product_id, 
  MIN(
    IF(
      cscart_product_prices.percentage_discount = 0, 
      cscart_product_prices.price, 
      cscart_product_prices.price - (
        cscart_product_prices.price * cscart_product_prices.percentage_discount
      )/ 100
    )
  ) AS price 
FROM 
  cscart_product_prices 
WHERE 
  cscart_product_prices.product_id IN (
    104633, 104632, 104631, 104630, 104629, 
    104628, 104627, 104626, 104625, 104624, 
    104623, 104622, 104621, 104620, 104619, 
    104618, 104617, 104616, 104615, 104614
  ) 
  AND cscart_product_prices.lower_limit = 1 
  AND cscart_product_prices.usergroup_id IN (0, 1) 
GROUP BY 
  cscart_product_prices.product_id

Query time 0.00053

Explain
id select_type table type possible_keys key key_len ref rows Extra
1 SIMPLE cscart_product_prices range usergroup,product_id,lower_limit,usergroup_id usergroup 9 40 Using where

Result

product_id price
104614 1.69000000
104615 1.69000000
104616 1.69000000
104617 1.69000000
104618 1.69000000
104619 1.69000000
104620 1.69000000
104621 1.69000000
104622 1.69000000
104623 1.69000000
104624 1.69000000
104625 1.69000000
104626 1.69000000
104627 1.69000000
104628 1.69000000
104629 1.69000000
104630 1.69000000
104631 1.69000000
104632 1.69000000
104633 8.99000000