You probably have a a table of Payors and a relationship where 1 Payor has many donations.
You can make a summary field on that relationship to total up all the donations.
Then look that up down to the donations table.
Then you can trigger the action when the cumulative donations is > x.
But the you will also need to know if the task has been created in order to not trigger the creation of the task for every subsequent donation as well. Assuming that these tasks are also children of the Payors, then you can also make a summary field to count up the # of that task type and also look that up down to the donations table so as to prevent the Action from creating a task that has already been created.
