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 (
    105398, 105397, 105396, 105395, 105394, 
    105393, 105392, 105391, 104328, 104286, 
    104277, 104276, 104275, 104273, 104272, 
    104271, 104270, 104251, 104250, 104249
  ) 
  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 0.00068

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
104249 1.69000000
104250 1.69000000
104251 1.69000000
104270 1.69000000
104271 1.69000000
104272 1.69000000
104273 1.69000000
104275 1.69000000
104276 1.69000000
104277 1.69000000
104286 1.69000000
104328 1.69000000
105391 1.69000000
105392 1.69000000
105393 1.69000000
105394 1.69000000
105395 1.69000000
105396 1.69000000
105397 1.69000000
105398 1.69000000