The right answer

In comments to this post, DL offers up the best suggestion I've heard as to the NDP's response to the issue of MP expense audits:

I think the NDP should say it agrees to an audit of MPs and senators on condition that the Auditor-General also conduct a "value for money" audit of the Privy Council Office and the Prime Ministers Office - both massive wastes of taxpayers money!

Now, I'm not entirely sure off hand as to what elements of PCO and PMO spending are already subject to what types of audits. And the NDP's scope for bargaining would be limited to the House of Commons since it can't speak for a Senate which doesn't include any NDP members - though any move in one chamber would presumably be followed in the other.

Those quibbles aside, it would make all the sense in the world for the NDP to push for greater accountability for government expenses in addition to the proposed MP audit, rather than playing into the narrative that the lone issue worth addressing involves Parliament rather than government agencies. And while I wouldn't expect the Harper Cons to ultimately accept such a proposal, there's still plenty of value in suggesting it - particularly if a Con refusal to see greater government accountability can be contrasted against eventual all-party agreement on MP expenses.