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 (
    104602, 104601, 102467, 102466, 102192, 
    102058, 102039, 102037, 102035, 102034, 
    104446, 102187, 102001, 101996, 101995, 
    102025, 102023, 102022, 102021, 102020
  ) 
  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 0.00077

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
101995 1.69000000
101996 1.69000000
102001 1.69000000
102020 1.69000000
102021 1.69000000
102022 1.69000000
102023 1.69000000
102025 1.69000000
102034 1.69000000
102035 1.69000000
102037 1.69000000
102039 1.69000000
102058 1.69000000
102187 1.69000000
102192 1.69000000
102466 1.69000000
102467 1.69000000
104446 1.69000000
104601 1.69000000
104602 1.69000000