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 (
    105381, 105380, 105379, 105378, 105377, 
    105376, 105375, 105309, 105308, 105307, 
    105306, 105305, 105304, 105303, 105302, 
    105301, 105300, 105299, 105298, 105297
  ) 
  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 0.00049

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
105297 8.99000000
105298 1.69000000
105299 1.69000000
105300 1.69000000
105301 1.69000000
105302 1.69000000
105303 1.69000000
105304 1.69000000
105305 1.69000000
105306 1.69000000
105307 1.69000000
105308 1.69000000
105309 8.99000000
105375 1.69000000
105376 1.69000000
105377 1.69000000
105378 1.69000000
105379 1.69000000
105380 1.69000000
105381 1.69000000