David Burkehead:

This is just simple math. People compare some “big ticket” item with “small ticket” items and don’t mention how the very large numbers of those small ticket items add up, or how very little the large ticket item would really stretch among the many to whom those small ticket items apply.

So when someone says “if we can afford X, then surely Y isn’t too expensive” take a closer look. Just how much of those “Y” do we have to buy and how much is the total cost?

Related: Math Forum audio and video.