Interpreting an execution plan
Total cost is almost always not reliable for things like this

I would use STATISTICS IO and STATISTICS TIME instead

run both queries and look at the difference in reads

SET STATISTICS IO ON
select * from [User] where FirstName ='John'
select * from [User] where Username = 'johndoe
SET STATISTICS IO OFF


and you can also look at the time by using this

SET STATISTICS TIME ON
select * from [User] where FirstName ='John'
select * from [User] where Username = 'johndoe'
SET STATISTICS TIME OFF