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 (
    37261, 37189, 37186, 37157, 58968, 40532, 
    40347, 40346, 40345, 40344, 37060, 
    40530, 40524, 40523, 40522, 40521, 
    40520, 40354, 36734, 36533
  ) 
  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 0.00062

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
36533 1.69000000
36734 1.69000000
37060 1.69000000
37157 1.69000000
37186 1.69000000
37189 1.69000000
37261 1.69000000
40344 1.69000000
40345 1.69000000
40346 1.69000000
40347 1.69000000
40354 1.69000000
40520 1.69000000
40521 1.69000000
40522 1.69000000
40523 1.69000000
40524 1.69000000
40530 1.69000000
40532 1.69000000
58968 1.69000000