Execution plan percentages don't add up to 100%
The sum of operator costs is more than 100% in execution plan is a known bug and is closed as by design!

Posted by Microsoft on 11/17/2010 at 5:28 PM
Thanks for taking the time to file this observation and simple repro. The strange cost percentage values that you observed are an aritifact of the specific structure of the query plan that are a bit confusing but ultimately do make sense. They will not adversely affect the running of the query in any way.
The concatenation operator has two children - the table scans. However the server expects that it will not be necessary to execute the second table scan. This is because it expects to find a row from the first table scan which would satisfy the &quot;exists&quot; part of the query. Therefore it does not include the cost of the second table scan when computing the total cost of the subtree rooted at the concatenation. However it still estimates and reports the cost of the second table scan, which the management tool the computes as a percentage of the total query cost as if it will be executed.
In this case of course, it would be necessary to run the second table scan since the first is empty. However the engine operates on the conservative assumption that there will always be at least one row (which may only have been added after the query was compiled).
Campbell
SQL Development

Aaron Bertrand filed a similar bug - SSMS : Execution plan sometimes exceeds 100%

Posted by Microsoft on 5/28/2008 at 11:58 AM
Thanks again for bringing this to our attention. This anomaly arise in the presence of &quot;row goals&quot; and the concatenation operator. Row goals is the term for when a subtree (perhaps the entire query) is not required to return all possible rows. This most commonly happens when the query has a &quot;top&quot; clause although there are other causes. The anomaly arises because we do not expect to execute the second child of the Concatenation (because we expect the first child to provide all the required rows). Those additional children are costed to assume they return a single row. The only alternative would be to give them 0 cost which would fix the anomaly but probably create other confusions and would lead the optimizer to not care about the plans - we don't want to do that just in case they are executed.
Campbell Fraser, SQL Development.

If you want to understand how plan costing works, Paul White explains it at his best here.
From the query processor team - What’s this cost?
General guidelines for better performance -

Have proper indexes &amp; up-to-date stats.
Fetch data that is required ONLY along with relevant columns.
Have proper schema.table with proper indexes &amp; datatypes
Use statistics time, IO before and after you make changes to measure the gain.
Get help / guidance from experts at this site :-)

That's why I use SQL Sentry's FREE Plan Explorer