The results of the execution time directly contradict the results of the Query Cost, but I'm having difficulty determining what "Query Cost" actually means.
Query cost
is what optimizer thinks of how long your query will take (relative to total batch time).
The optimizer tries to choose the optimal query plan by looking at your query and statistics of your data, trying several execution plans and selecting the least costly of them.
Here you may read in more detail about how does it try to do this.
As you can see, this may differ significantly of what you actually get.
The only real query perfomance metric is, of course, how long does the query actually take.