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 (
    110550, 110388, 110332, 110331, 110329, 
    110328, 110419, 110389, 110424, 110423, 
    110391, 110390, 110349, 110696, 110386, 
    110358, 110357, 110354, 110351, 110350
  ) 
  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 0.00063

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
110328 1.69000000
110329 1.69000000
110331 1.69000000
110332 1.69000000
110349 1.69000000
110350 1.69000000
110351 1.69000000
110354 1.69000000
110357 1.69000000
110358 1.69000000
110386 1.69000000
110388 1.69000000
110389 1.69000000
110390 1.69000000
110391 1.69000000
110419 1.69000000
110423 1.69000000
110424 1.69000000
110550 1.69000000
110696 1.69000000