The payment on a $300,000 loan would depend on the interest rate and the terms (length of repayment). Let's just use worst case scenario 8.25% at 10 year repayment terms...monthly payment would be about $3700 or about $45,000 per year. So if you assume you will make $200,000 a year then you really only make about $150,000 per year after loan repayment...still pretty good.
You can think about borrowing money for medical school as an investment. If there was some investment on wallstreet that would allow someone to borrow $300,000 and then guarantee a minimum of $100,000 per year paid back for the next 20 or 30 years, every investor would want in. Your MCAT score, good grades, and hard work in medical school and residency give you exclusive right to that investment.
Also think about how many people that earn far less than $200,000 and live in houses worth more than $300,000 (that certainly don't appreciate at $150,000+ per year).
$300,000 is a lot of money to borrow, no doubt, but if you don't need to live like a millionaire you can probably do fine (just be sure to take out a large insurance policy).