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 (
    47803, 47802, 47801, 47800, 47799, 47798, 
    47797, 47796, 47795, 47794, 47793, 
    47792, 47791, 47790, 47789, 47788, 
    47787, 47786, 47785, 47784
  ) 
  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 0.00053

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
47784 1.69000000
47785 1.69000000
47786 1.69000000
47787 1.69000000
47788 1.69000000
47789 1.69000000
47790 1.69000000
47791 1.69000000
47792 1.69000000
47793 1.69000000
47794 1.69000000
47795 1.69000000
47796 1.69000000
47797 1.69000000
47798 1.69000000
47799 1.69000000
47800 1.69000000
47801 1.69000000
47802 1.69000000
47803 1.69000000