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 (
    73795, 73794, 73793, 73792, 73791, 73790, 
    73789, 73788, 73787, 73434, 73433, 
    73432, 73431, 73430, 73429, 73428, 
    73427, 73426, 73425, 73424
  ) 
  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 0.00060

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
73424 1.69000000
73425 1.69000000
73426 1.69000000
73427 1.69000000
73428 1.69000000
73429 1.69000000
73430 1.69000000
73431 1.69000000
73432 1.69000000
73433 1.69000000
73434 8.99000000
73787 1.69000000
73788 1.69000000
73789 1.69000000
73790 1.69000000
73791 1.69000000
73792 1.69000000
73793 1.69000000
73794 1.69000000
73795 1.69000000