The Simpsons survie on only Homers salary which can't be that great. Still, they own two cars and live in a two-story four bedroom detached house in an average size American city. How could they afford this?
Aside from the obvious answer: It's a cartoon--we can make some assumptions with real-world data.
The average Nuclear Power Plant Operator's salary is 72.5k a year. The average house price in Springfield, oh...let's say Missouri is 107k. Let's say that this is a really nice house for the area and they paid $150k. Let's figure they got in on an OK interest rate which would put them somewhere in the range of $1300/month for their mortgage payment? Given that their cars are obviously well used, Marge does most of the cooking (not a lot of eating out expenses), neither kid goes to private school, and they all seem to have a very tiny wardrobe, I'm thinking they're getting by fairly OK paycheck-to-paycheck.
And living paycheck-to-paycheck is the modern american dream. :)
In order for the Simpson family to purchase the home, Abraham Simpson sold his old house and wrote Homer a check for $15,000, allowing him to pay the down payment on the house. In No Loan Again, Naturally it is revealed that the Simpsons are unable to afford their mortgage anymore, due to Homer constantly loaning money against the house, which causes Ned Flanders to buy the house for $101,000 and rent it to the Simpsons.