To test the API and familiarize yourself with the product, you can easily sign up for the Free Plan on exchangeratesapi.net.
Once your needs or those of your application exceed 1000 API requests per month, or you require more advanced API functionalities (such as for running a currency converter, accounting & invoicing, financial planning, estimates, shopping carts, etc.), it's recommended to consider one of the paid Subscription Plans. These plans start at a startup-friendly price of $4.00 per month.