If you don't buy one coffee, or put a 20 dollar note in a book one month. Then you're fine. And if you have to use EC2, just use a t2.micro or a raspberry pi on your desk.
But really the first lesson you should learn in any cloud setup is Billing Alarms :)
If you're doing ML or CV work then it's probably cheaper to build on the desktop and port to cloud once you understand what the workloads are.