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 (
    103713, 103712, 103711, 103710, 103709, 
    103708, 103707, 103706, 103705, 103704, 
    103703, 103656, 103655, 103654, 103653, 
    103652, 103651, 103650, 103649, 103648
  ) 
  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 0.00082

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
103648 1.69000000
103649 1.69000000
103650 1.69000000
103651 1.69000000
103652 1.69000000
103653 1.69000000
103654 1.69000000
103655 1.69000000
103656 8.99000000
103703 1.69000000
103704 1.69000000
103705 1.69000000
103706 1.69000000
103707 1.69000000
103708 1.69000000
103709 1.69000000
103710 1.69000000
103711 1.69000000
103712 1.69000000
103713 1.69000000