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 (
    100759, 100758, 100757, 100756, 100755, 
    100727, 100648, 100647, 100645, 99651, 
    98893, 92945, 100368, 100629, 100176, 
    100627, 100625, 100624, 100623, 100596
  ) 
  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 0.00055

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
92945 1.69000000
98893 1.69000000
99651 1.69000000
100176 1.69000000
100368 1.69000000
100596 1.69000000
100623 1.69000000
100624 1.69000000
100625 1.69000000
100627 1.69000000
100629 1.69000000
100645 1.69000000
100647 1.69000000
100648 1.69000000
100727 1.69000000
100755 1.69000000
100756 1.69000000
100757 1.69000000
100758 1.69000000
100759 1.69000000