And remember, those costs are just estimated values. They don't represent a literal real-world performance metric. It's just a measure of what the optimizer thinks a given operation will cost based on the operation and the statistics presented to the optimizer.
"The credit belongs to the man who is actually in the arena, whose face is marred by dust and sweat and blood"
- Theodore Roosevelt
Author of:
SQL Server Execution Plans
SQL Server Query Performance Tuning