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 (
    103750, 103749, 103748, 103747, 105270, 
    105269, 105268, 105267, 105266, 105265, 
    105264, 105263, 105218, 105217, 105216, 
    105215, 105214, 105213, 105212, 105211
  ) 
  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 0.00059

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
103747 1.69000000
103748 1.69000000
103749 1.69000000
103750 1.69000000
105211 1.69000000
105212 1.69000000
105213 1.69000000
105214 1.69000000
105215 1.69000000
105216 1.69000000
105217 1.69000000
105218 8.99000000
105263 1.69000000
105264 1.69000000
105265 1.69000000
105266 1.69000000
105267 1.69000000
105268 1.69000000
105269 1.69000000
105270 8.99000000