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 (
    111741, 111740, 111739, 111738, 111737, 
    111736, 111735, 111734, 111733, 111732, 
    111689, 111836, 111835, 111685, 111677, 
    111676, 111675, 111674, 111771, 111770
  ) 
  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 0.00055

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
111674 1.69000000
111675 1.69000000
111676 1.69000000
111677 1.69000000
111685 1.69000000
111689 1.69000000
111732 1.69000000
111733 1.69000000
111734 1.69000000
111735 1.69000000
111736 1.69000000
111737 1.69000000
111738 1.69000000
111739 1.69000000
111740 1.69000000
111741 1.69000000
111770 1.69000000
111771 1.69000000
111835 1.69000000
111836 1.69000000