The trouble with Blackford and his fellow SNP MP's is perhaps not so much how much they cost, but of the value for money they represent.
SNP MP's especially have re-written the definition of an 'opposition MP'. the purpose of an opposition MP or party is to attempt to negotiate with the Government to try and win some of modification or concession that benefits the portion of the electorate they represent. The SNP MP's are by no means the only ones at it, but they make the biggest meal out of it, in that they interpret the role of being an opposition MP or party as simply criticising and voting against anything and everything the Governent proposes doing, up to and including opposing things that if they themselves were in Government they'd be trying to enact themselves.
Their interpretation of 'opposition' is that it is literal and must always be adhered to on principle whatever the cost, which is their choice to make. What gets me though is, that the folk who vote in SNP MP's do so when those MP's operate in such a way that makes it impossible for them to achieve anything for their constituents, only blow their own trumpet and pull a salary for doing it.