It should read "my costs were a bit higher". A plural subject requires a plural verb. You should knott butcher the language.. Carry on with your twittering about washing detergent.
Actually Bubba, no need for a plural noun whatsoever..." my cost was a bit higher " will not only suffice...it is proper. Shall we now discuss dangling participles? bwaaaaaaaaaaa
I wasn't going to debate English with a retired no-it-all
Sometimes it just isn't worth bickering about the small stuff.... .