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 (
    102019, 102018, 102017, 102016, 102015, 
    102014, 102013, 102012, 102011, 102010, 
    102009, 102008, 102007, 102006, 102005, 
    102004, 102003, 102002, 102031, 102030
  ) 
  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 0.00062

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
102002 1.69000000
102003 1.69000000
102004 1.69000000
102005 1.69000000
102006 1.69000000
102007 1.69000000
102008 1.69000000
102009 1.69000000
102010 1.69000000
102011 1.69000000
102012 1.69000000
102013 1.69000000
102014 1.69000000
102015 1.69000000
102016 1.69000000
102017 1.69000000
102018 1.69000000
102019 1.69000000
102030 1.69000000
102031 1.69000000