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 (
    103655, 103654, 103653, 103652, 103651, 
    103650, 103649, 103648, 103647, 103646, 
    103645, 103644, 103643, 103642, 103641, 
    103640, 103639, 103638
  ) 
  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 0.00078

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 36 Using where

Result

product_id price
103638 1.69000000
103639 1.69000000
103640 1.69000000
103641 1.69000000
103642 1.69000000
103643 1.69000000
103644 1.69000000
103645 1.69000000
103646 1.69000000
103647 1.69000000
103648 1.69000000
103649 1.69000000
103650 1.69000000
103651 1.69000000
103652 1.69000000
103653 1.69000000
103654 1.69000000
103655 1.69000000