I use Campaign Monitor for one "client" -- my own company. But even for that I cannot figure out how much I've spent. Is there somewhere on the site where there's a total per client, or even a total spent overall?
We don't have a total figure, but we do store all your invoices under 'billing history' in your account settings, so you can go back and review them.
We'll add a feature request to our list here for some summary figures.