Although these are two different words that mean two different things, they are spelled very similarly, and I think that’s how the confusion creeps in. Having said that, even though the a and e are not next door to one another on the keyboard (I had to check that: I touch-type and most of the letters have worn off my keyboard through over-use) this one does come up fairly frequently.
Expensive means of high cost – either in monetary or some other terms. An expensive training course costs a lot, an expensive manoeuvre might cost the army dear in terms of people or equipment.
Expansive means covering a wide area, or extensive (although I don’t see extensive mixed up with expensive, which you would kind of expect!). It also means relaxed, communicative and genial, when used to describe a person. “The grounds of the castle are expansive”.
So you may have grounds which are both expansive, as in large, and expensive in terms of their cost of upkeep. But there’s going to be no more mixing up of the two words, now, is there?