... I have over 100K of loan principal myself, which I think is "substantial." However, I have the good fortune of paying a very low interest rate (1.3%) and low monthly payments. Particularly with my wife's income on top of it, we're paying well under 5% of our income for debt service. We've been able to afford to buy a house and we both contribute to our retirement accounts and our savings.
Also with a 1.3 percent interest rate, by simply paying the minimum payments on the 30 year plan, inflation actually shrinks my loan principal for me (last year the CPI was 2.2 percent annualized).
So, while my loan principal is definitely significant, my debt service is actually fairly small and fairly manageable - and happily, I just found out I qualify for loan repayment since I work in the public sector. With any luck, in a few more years, with a combination of inflation and loan repayment programs, I may make significant headway on shrinking that principal.
On the other hand, I can see how even a loan principal of even just 10K can be significant, say, if it's run up on credit cards or through payday loans. If the interest is 10, 20, 30 percent or more, the debt service can definitely be difficult to manage.... which is why I asked about the current crop of pro-school students. While 1.3 percent interest is manageable for me, I can see how I would have struggled with 6.8 percent, particularly in the beginning....
Would I have preferred going to a funded school? Absolutely - but the fact was I was not in a place in my life where I would have been able to attend a school for many years in a state far away from my family and possibly my current wife - and how do you attach a dollar value to costs like that?
So, that's what I chose, and now I pay a few hundred bucks a month because of that choice. So far, it's been worth it.