MD-PhD programs are unique in that they require student to remain in school for 7-9 years. Once you graduate the idea is for you to become a translational researcher. Not exactly a high paid field. How do you manage this? Well, unlike regular medical students most MD-PhD's are given some sort of funding. However, not all programs are created equal and not all offer the same sort of funding. Always make sure you understand your funding before you sign on to join a program!
Medical Students typically pay high tuition and most take out loans to do so. There are a few excellent scholarships but they are few and far between and you cannot count on obtaining one. Medical students should make sure they are comfortable with the financial aid department at their school and fully understand all loans and repayment options. Once they exit medical school they can usually defer their loans repayment through residency (interest variable depending on the type of loan and consolidation you do), some like to pay as much as they can off as soon as possible. After that they enter the workforce and repay those loans as fast as possible. There are some loan repayment programs for people who work in under served areas or at the NIH. These usually do not cover all your loans but are a great relief as people who enter those professions are usually not receiving a large salary. Not having had to deal with this sort of thing, I do not know all the details.
The best type of MD-PhD funding is the type found at my institution and at all other MSTPrograms (more on the initials in later posts). Your tution is waived for all years by the school and you receive a stipend of about 25,000 - 30,000 a year. Your health insurance is covered as well and sometimes you get an aditional allowance for travel to conferences or lab materials. If you drop the program you are not responsible to pay back tuition, however this is generally considered a very terrible thing to do and the "two and screw" (get two years of medical school for free and then drop the program before the PhD) is considered the most vile thing anyone can do. You are cheating someone who would have become a medical researcher out of their spot in the program and wasting tax payer money. Most people who choose to drop the program do so for compelling personal reasons. Dropping or not finishing is extremely rare at my institution. (drop rate is another great thing to ask about when interviewing at programs). Sounds great right? Well, don't get ahead of yourself. Your MD classmates will be in their well paying jobs by the time you hit residency and you don't actully gain much by not going the MD route with loans. This is not a great way to get a fre MD.
Other non MSTPrograms are not required to follow this funding plan. The next most common plan is one in which your tution is waved and you receive a stipend (variable based on program) however you are responsible for back tuition should you drop. Not a terrbile option. Just check that stipend levels are enough to live by in the area.
Rarely a program will tell you to pay for the first two years of medical school upfront or with loans and upon completion of your PhD you will receive this money back. They have placed it in a seecure account for you. They do this because they see it as an incentive to complete the program. I would avoid this type of program if you have other options. If you must, at least ask them how they plan on handling any interest accumulated on your loans while you are in graduate school.
Then there are the programs that have no funding at all. This is extremely rare in the US. It does happen occasionally in Canada and other countries. There is significant hardship involved in this and I would suggest that if this is your only option you should consider not doing the dual degree or doing them separately with a few years of working and cash accumulation in between. Unless of course you have an independent unlimited cash supply.