Yes, as soon as you provision a load generator, costs will begin to accrue. Additionally, you will be charge a nominal monthly fee for maintaining the account. You will have to register a credit card when you create the account. A single load generator will cost somewhere between $0.13 to over $2.00 per hour depending on the number of CPUs, Memory, and features selected. My monthly charge account fee is around $5.00. Your charges will vary. The above is just a ball park figure.
Keep in mind that once you provision a load generator, charges will occur. Always remember to close your test scenarios when testing is done. A good idea is to check in the Amazon EC2 Dashboard to ensure that your instances have stopped.