a stadium built 'at the taxpayers' expense' = a stadium built 'at taxpayer expense' [=with money from taxes]
These items were imported 'at' great/considerable/enormous 'expense'. [=it cost a lot to import them]
My new computer was a business 'expense'. [=something I had to buy in order to do business]
(..) - I am betrayed. - Betrayed? - By whom? - By the King. He has made himself King of Mercia, at my expense. And now I am nothing. Ecbert did this to you? Dear God. - What will you do? - I cannot stay here. I will take Magnus and I will go away. (..)2016 TvShow: Vikings Title: Portage Season: 4 Episode 8