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 (
    60679, 60678, 60677, 60676, 60675, 60674, 
    60673, 60672, 60671, 60670, 60669, 
    60668, 60667, 60312, 60311, 60310, 
    60309, 60308, 60307, 60306
  ) 
  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 0.00063

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
60306 1.69000000
60307 1.69000000
60308 1.69000000
60309 1.69000000
60310 1.69000000
60311 1.69000000
60312 8.99000000
60667 1.69000000
60668 1.69000000
60669 1.69000000
60670 1.69000000
60671 1.69000000
60672 1.69000000
60673 1.69000000
60674 1.69000000
60675 1.69000000
60676 1.69000000
60677 1.69000000
60678 1.69000000
60679 1.69000000