What a substainable income in the United States of America is?

This varies wildly based on where and how you live. For instance, you may struggle making $100k/year in San Francisco or NYC, or live quite comfortably on half that much in many smaller cities. When I graduated from college, I was making less than $20k and could afford to pay all of my bills because I had two roommates in a small apartment in a small town. I also had no student loan debt.

My advice is to pursue a field that interests you and create a lifestyle that you can afford. Don't take out large loans for a private or for-profit education that does not tend to lead to a stable job with a high salary.

It depends on a lot of factors. Where do you plan to live? Major cities like Washington, DC have a high cost of living and require a higher income. What kind of education do you plan to pursue? The cost of college and student loans should be factored in to your target income.

Based on my personal experience, a good income to target right out of undergrad is 40-50,000 per year. This is do-able if you pick a course of study designed to make you employable upon graduation. For instance, my company has an internship-to-hire program designed for people who have just graduated from college with a degree in economics, finance, math, or computer science. Upon successful completion of the internship most people make more than $50K/year in salary.