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 (
    80587, 80567, 80015, 80010, 80009, 80004, 
    80472, 103007, 80448, 80001, 80272, 
    80080, 80079, 80078, 80077, 80076, 
    80075, 80074, 80073, 80072
  ) 
  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 0.00097

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
80001 1.69000000
80004 1.69000000
80009 1.69000000
80010 1.69000000
80015 1.69000000
80072 1.69000000
80073 1.69000000
80074 1.69000000
80075 1.69000000
80076 1.69000000
80077 1.69000000
80078 1.69000000
80079 1.69000000
80080 1.69000000
80272 1.69000000
80448 1.69000000
80472 1.69000000
80567 1.69000000
80587 1.69000000
103007 1.69000000