What’s the most I could be billed in a month?

Your maximum is the plan fee plus any overflow already incurred; pay-as-you-go also includes your remaining cap headroom.

With cap-and-hold (or shutoff), the figure is your plan fee plus any overflow already incurred this month. With pay-as-you-go, it also includes whatever remains under the overflow cap you set. Your dashboard shows this “max possible bill” figure directly. On an annual plan the fee part is the full annual price — the honest worst case for your renewal month, not a monthly average. (Usage-based build/compute, if you use it, draws your prepaid balance separately and isn’t part of that number; a complimentary plan shows $0.)