When you filter rows in Numbers, any totals or other footer calculations will still be based on all of the rows, not just the ones visible. You can change that by using functions like SUMIF that match the filter. But then you need to adjust your formulas and filters every time so they are the same. However, you can use a hidden column and a single-cell separate table to make this quick, easy and useful.
Download the example: FilterSums.numbers.zip