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 (
    119395, 119394, 119393, 119392, 119391, 
    119390, 119389, 119388, 119387, 119386, 
    119385, 119384, 119383, 119382, 119381, 
    119380, 119379, 119378, 119377, 119376, 
    119375, 119374, 119373
  ) 
  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 0.00067

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 46 Using where

Result

product_id price
119373 1.69000000
119374 1.69000000
119375 1.69000000
119376 1.69000000
119377 1.69000000
119378 1.69000000
119379 1.69000000
119380 1.69000000
119381 1.69000000
119382 1.69000000
119383 1.69000000
119384 1.69000000
119385 1.69000000
119386 1.69000000
119387 1.69000000
119388 1.69000000
119389 1.69000000
119390 1.69000000
119391 1.69000000
119392 1.69000000
119393 1.69000000
119394 1.69000000
119395 1.69000000