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 (
    112148, 111842, 111841, 111840, 111839, 
    112382, 111756, 111755, 111754, 111753, 
    111751, 111750, 111749, 111748, 111747, 
    111746, 111745, 111744, 111743, 111742
  ) 
  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 0.00070

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
111742 1.69000000
111743 1.69000000
111744 1.69000000
111745 1.69000000
111746 1.69000000
111747 1.69000000
111748 1.69000000
111749 1.69000000
111750 1.69000000
111751 1.69000000
111753 1.69000000
111754 1.69000000
111755 1.69000000
111756 1.69000000
111839 1.69000000
111840 1.69000000
111841 1.69000000
111842 1.69000000
112148 1.69000000
112382 1.69000000