Are the cost percentages in this SQL Server plan over 100% for a valid reason?

The visual cost estimator is crap. This sort of stuff happens all the time. Just go with the highest ones are the most expensive and attack those ones first.

I'd been also curious about why sometimes some cost is displayed as 100%, 200%, 300%...and even more. After analyse xml file of query plan, i got it.

The cost percentage = my EstimatedTotalSubtreeCost / parent node's EstimatedTotalSubtreeCost

For example, your query plan shows Clustered Index Insert take cost 914%, to understand how it calculate this percentage, 1. Move mouse to `Clustered Index Insert` to show popup, you can see the cost `EstimatedTotalSubtreeCost`, e.g, 0.2 2. Move mouse to this node's parent node `COND WITH QUERY`, check the popup to get `EstimatedTotalSubtreeCost`, e.g, 0.0218818 3. Calculate 0.2/0.0218818 = 914%, this is the cost percentage displayed in `Clustered Index Insert` popup