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 (
    110359, 110330, 110326, 110421, 110420, 
    110418, 110417, 110416, 110415, 110414, 
    110413, 110412, 110411, 110410, 110409, 
    110408, 110407, 110406, 110405, 110404
  ) 
  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
110326 1.69000000
110330 1.69000000
110359 1.69000000
110404 1.69000000
110405 1.69000000
110406 1.69000000
110407 1.69000000
110408 1.69000000
110409 1.69000000
110410 1.69000000
110411 1.69000000
110412 1.69000000
110413 1.69000000
110414 1.69000000
110415 1.69000000
110416 1.69000000
110417 1.69000000
110418 1.69000000
110420 1.69000000
110421 1.69000000