So I did some more quick Google-ing and I think I got it now...

1. Create a table in Word - the cells are identified the same as they are in Excel, Word just won't show it explicitly (A1, A2, B1, B2, etc.)

2. Use the Text Form Field to insert fields where your numbers will go. Make sure you select "Calculate on Exit" in the Form Field Options for each one. This allows auto-calculations to occur where you will have your function.

3. Table-->Formula or Insert-->Field-->Formula in the cell where you want your final calculation to be done.

Full tutorial here

This only works within single tables, not across multiple tables, which still doesn't completely solve my problem.