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 (
    101801, 101800, 101799, 101798, 101797, 
    101796, 101795, 101794, 101793, 101792, 
    101791, 101790, 101789, 101788, 101787, 
    101786, 101785, 101784, 101783, 101782
  ) 
  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 0.00058

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
101782 1.69000000
101783 1.69000000
101784 1.69000000
101785 1.69000000
101786 1.69000000
101787 1.69000000
101788 1.69000000
101789 1.69000000
101790 1.69000000
101791 1.69000000
101792 1.69000000
101793 1.69000000
101794 1.69000000
101795 1.69000000
101796 1.69000000
101797 1.69000000
101798 1.69000000
101799 1.69000000
101800 1.69000000
101801 1.69000000