This might not be the answer you want to hear but this is the best answer we can provide you.
Google Maps Platform offers a free $200 monthly credit for Maps, Routes, and Places once you enable billing in your project. With the $200 monthly credit, the vast majority of customers find their use cases are completely free. You won't be charged until your usage exceeds $200 in a month.
You can avoid surprises on your bill by doing the following:
- Set a budget alert, to track how your spend is growing toward a particular amount. Setting a budget does not cap API usage, it only alerts you when your spend amount gets near the specified amount.
- Cap your daily API usage, to manage your cost of use of billable APIs. By setting caps on requests per day, you can limit your spend. Use a simple equation to determine your daily cap depending on how much you want to spend. For example: (Monthly spend / price per each )/30 = requests per day cap (for one API). Note that your implementation may use multiple billable APIs, so adjust your equation as needed. Remember, a $200 USD Google Maps Platform credit is available each month, so be sure to factor that into your calculation.
Here's the pricing for the Directions API.
Hope you find it helpful and good luck on your project.