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 (
    103925, 103924, 103923, 103922, 103921, 
    103920, 103919, 103918, 103917, 103916, 
    103915, 103914, 103913, 103912, 103911, 
    103910, 103909, 103908, 103907, 103906
  ) 
  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 0.00066

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
103906 1.69000000
103907 1.69000000
103908 1.69000000
103909 1.69000000
103910 1.69000000
103911 1.69000000
103912 1.69000000
103913 1.69000000
103914 1.69000000
103915 1.69000000
103916 1.69000000
103917 1.69000000
103918 1.69000000
103919 1.69000000
103920 8.99000000
103921 1.69000000
103922 1.69000000
103923 1.69000000
103924 1.69000000
103925 1.69000000