DD-WRT works well on a compatible router and TBB is great for monitoring usage on a machine but I don't see how they can work together to tell you how much is going through the router.

I like most people need a router that can do the traffic monitoring to everything in my house (3 desktops, 2 laptops, 2 phones, 1 tablet + guest's equipment) so I can confirm what my provider is saying I use. I am an untrusting sort of person when there are no rules and regs for how a company measures bandwidth.