How can I tell what I'll be charged next month? What tier am I on?

First of all you can see the SellerMetrics pricing plan here.

Since the pricing is a bit dynamic, you can more or less approximate what your subscription tier will be. You can just go to the dashboard, select the date filter. On the date filter module, enter "30" on the part that says "days up to today" and click Apply.

The resulting Spend ($) figure will be your approximate subscription tier on your next invoice. This will give you an estimate, but if you are on the edge of the pricing tier ad spend-wise, there is a chance of a tier increase.