Your SpyFu Pro Plan or Teams Account includes a credit of 10,000 units. Anything over that is charged to the credit card on file for your account at a cost of $0.50 for every 1000 extra units.
You can see your API usage by month at https://www.spyfu.com/account/api
For every API request made, you get a number of rows returned. We will use the "rows returned" to make the calculations and call them units.
Here's a break down.
In this example, I've gotten back 22,333 rows across all 4 API endpoints that I tried.
You have a base of 10,000 units included, so that leaves 12,333 units as the overage.
Overage is charged in increments of 1000, so it is $0.50 x 12.33. This makes a final API cost of $6.17 for the month.