It marked 'euro'in place of 'euros' as an error but both are correct ways of forming the plural in English. Should've been accepted.
The sentence is not a proper question. It is supposed to be a statement. The question mark is only there to emphasize the disbelief of the speaker regarding the high prize. To make it so that it would be a real question (Was, kostet der Stift 70 €?) sounds more like what you would say when you couldn't hear what the prize was when you asked the first time. This you would say if you weren't sure of the prize itself, not if you wanted to point out that 70€ for a pen is ridiculous.
The structure: Was, [statement]? Can be used in nearly every context.