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?
You can find more troublesome pairs here and the index to them all so far is here.
LibroEditing proofreading, editing, transcription, localisation 
Readers Cafe
May 28, 2012 at 10:44 am
I really like the article. Nice topic about how similar words can have different meaning. All the more reason to believe that English is a fascinating language.
LikeLike
Liz at Libro
May 28, 2012 at 11:19 am
Thank you – you might want to look at the index to the Troublesome Pairs – I have done lots and lots of them over the year or so I’ve been writing them!
LikeLike
lindaproofreads
August 17, 2012 at 7:06 pm
I was just coming on here to suggest expend/expand as a troublesome pair. Another that has cropped up a few times in recent days has been intention/intension. I must admit that I’ve never knowingly seen intension used in a sentence intentionally (!); it creeps in for intention and is probably confusing people because Word accepts it as a valid word.
LikeLike
Liz at Libro
August 20, 2012 at 5:33 pm
Thanks, Linda. No reason wey I can’t do expend and expand as well as expensive and expansive. I have a nice big list now; just have to write some up!
LikeLike