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 (
    109483, 109228, 109227, 109167, 109166, 
    108809, 108808, 108807, 108800, 108796, 
    108795, 109600, 108788, 108787, 108779, 
    108778, 108777, 108776, 109363, 109242
  ) 
  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 0.00056

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
108776 1.69000000
108777 1.69000000
108778 1.69000000
108779 1.69000000
108787 1.69000000
108788 1.69000000
108795 1.69000000
108796 1.69000000
108800 1.69000000
108807 1.69000000
108808 1.69000000
108809 1.69000000
109166 1.69000000
109167 1.69000000
109227 1.69000000
109228 1.69000000
109242 1.69000000
109363 1.69000000
109483 1.69000000
109600 1.69000000