Adding cost from multiple tables

Hi,

In one base I have the following:

1. One table that lists my clients (Current Clients)
2. One table that lists the client orders with cost per order (the Current Clients tallies the amount of units ordered but NOT cost)
3. One table the tallies cost per client (total and per month).

I need table 3 to tally the costs. I’m not sure how to do that. I’m a long-time Google Sheets users, but that experience isn’t helping here.

I know I can tally cost in table 1, but I want a separate table to tally cost.

Thanks.