Paid Workspaces can calculate numeric fields in List view to find their sum, average, or range!
What you'll need

Workspaces on the Unlimited Plan and above can calculate columns.

Everyone, including guests, can calculate columns.
Calculate columns
To calculate a column:

In any List view, click the + icon to show existing columns or add additional columns.

Hover over any numeric column to reveal the Calculate button at the bottom of it.

Select Sum, Average, or Range.

Click Calculate.
Column calculations will recognize any task filters that are currently applied. Tasks that are excluded with a filter will not be included in the column calculation.
This will work for both standard fields as well as numeric Custom Fields that you create.
Once you add a calculation, you'll see results for each group in your List view, as well as a calculation for the entire column.
Calculate an entire column
Eliminate task grouping to see one calculation for your entire column:

Click on a column header in a List view to open the column settings menu.

Select Sort entire column to remove any grouping applied to the List view and sort tasks by the selected field.

Select Sum, Average, or Range.

Click Calculate.
Updated