Pretty good question: I think most of the difference have been mentioned already especially higher quality woods and maybe better finish... but really in terms of sound I really recommend that you try both.
I'm convinced that overall quality has definitely improved in the Mexican factory and they do produce extremely good guitars now. I'm not a pro but I think that most of the cost difference now comes from 'the legend' itself: you pay more because it comes from the US, a great Fender has to come from the US somewhat, the origin of the brand.
Actually, I was looking for my first Telecaster US this summer. Since I got a new job with good income, I was ready to spend what it takes to get me a great one. Hours later well spent in music shops, playing and listening at many guitars (not only Fender by the way!!) I found it and guess what?
I went for a Mexican Telecaster 72'Thinline
, a beauty with the sound I was simply dreaming to find. No comparison even with US Tele or Strat I tried.
So my only advice is to trust your ears, your fingers, your feelings. A Mexican Fender can sound better or as well as US one for less money. You just have to look around and take your time. Good luck and have fun!!
PS: message for all Fender's Mexican techs:
God bless you Guys, I wish I can thank you all for that jewel that is the Tele 72'Thinline that I've just bought. Great job, the sound is just...my soul! For future productions, think about putting a Mexican flag sticker on the neck