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 (
    45227, 45226, 45225, 45224, 45223, 45222, 
    45221, 45220, 45219, 45218, 45217, 
    45216, 45215, 44881, 44880, 44879, 
    44878, 44877, 44876, 44875
  ) 
  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 0.00056

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
44875 1.69000000
44876 1.69000000
44877 1.69000000
44878 1.69000000
44879 1.69000000
44880 1.69000000
44881 8.99000000
45215 1.69000000
45216 1.69000000
45217 1.69000000
45218 1.69000000
45219 1.69000000
45220 1.69000000
45221 1.69000000
45222 1.69000000
45223 1.69000000
45224 1.69000000
45225 1.69000000
45226 1.69000000
45227 1.69000000