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 (
    8273, 10423, 9819, 9481, 8283, 9483, 
    9879, 9875, 9869, 10428, 8735, 8287
  ) 
  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 0.00044

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 product_id 3 12 Using index condition; Using where

Result

product_id price
8273 93.640000
8283 63.510000
8287 71.680000
8735 126.000000
9481 62.100000
9483 64.190000
9819 51.390000
9869 88.950000
9875 45.140000
9879 41.460000
10423 104.600000
10428 70.290000