Hi Anh,
Have a look at Filtered Values. Using them with a combination of Seat Hold Code and Seat Sold Flag would allow you to limit to just Seat Sold Flag = N and Seat Hold Code = (none) for Available or <> (none) for Held.
(SUM([Seat Current Base Amount]), [Seat Sold Flag], [Seat Hold Code])
Alternatively, the Seat Status field might be slightly easier, filtered to Held or to Available.
(SUM([Seat Current Base Amount]), [Seat Status])