Many Mexican Fenders have the same or similar price as the American-made versions. Some of the parts are also the same. Generally what's the difference?
What differs generally depends on the relative models and relative years. In general, an MIM Strat will have a body made of more pieces of wood (3 or even 4/5 pieces instead of 2), use poplar instead of alder for the body wood (especially for opaque finishes; trans finishes generally either have 3-piece alder bodies or an alder laminate), have cheaper pickups (ceramic magnets instead of alnico), and have more stamped metal parts instead of die-cast (string trees, bridge saddles). That's in addition to Mexican labor (by far the biggest discount in direct costs) and slightly more lax QA (when you buy an MIM, you definitely want to look at and play a few to make sure you get a good one; an MIA doesn't require AS MUCH of this, but you can get a bad MIA as well).
At least with basses, the MIAs have also generally had more experimentation with new electronics ideas, such as the S-1 tone circuit which allows you to switch tone cap values with the press of a button. The Highway-1 series Strats (basically an American-assembled MIM with 70's styling and a few other tricks) did a little of this (my H-1 has Greasebucket tone circuits which reduce the boominess you get when you roll off the tone), but usually when you buy an American Standard Strat you are getting exactly that.
Wood, electronics, pick-ups, basically all of the parts of the guitar can be different in Mexico and American made fenders, depending on model and build date.
The Made-In-America ones are higher quality, both in parts and build.
Made-In-Mexico ones are still Fender products, and quality instruments. The general complaint against Made-In-Mexico Fender's is the build quality is a bit hit or miss.
I wouldn't trade my 2008 MIM Strat for a Custom Shop rig. It plays and sounds just right for me. To my ears, has more of a classic sound/tone than the newer U.S. models. Of course, with so many options these days, what does a Strat sound like? And, why would you take a Strat and swap out pups, electronics? When you do that, it is no longer a Strat, IMO. Just a pet peeve of mine. Of course, I am obviously not a gear snob. I mean, is Eric Clapton playing a "real" Strat anymore? Nope. Once you start modding, it is no longer a "real" Strat. Or Tele. Or Jag. etc.
I have a 2009 MIM std. Tele. Fantastic w/sock pickups through a variety of amps. Only change I made was straplocks and a matte black pickplate...looks ultra-retro on arctic white! Also have a 2008 MIM std. Strat. It is the discontinued metallic 'Midnight Blue', my all time favorite Fender color. In some light it takes on a slight purple hue. Looks so tasty I want to take a bite out of it! Upgraded p-ups to Carvin AP11s.....better quack than stock! RWFB. Wouldn't trade either one of them for an MIA! Great action on both. Both strung with 11-50s because I switch b&f from acoustic quite a bit. Got strat new on a straight across trade for a jazz bass. Bought the tele on a whim cuz I was p/o'd at my then girlfriend! Now a year and a half later, still loving the tele, plus, now I gotta girlfriend whose not an insane, bipolar psycho!!
I have an 1996 Standard MIM Strat ($350) and I have to be honest and tell you this guitar sounds cheap and it's miles away from every Fender Standard Strat MIA that I've tried.
When I bought the guitar in 1997 a friend of mine bought a Squier Strat in the same week, half the price of mine. I was a kid and I was completelly ashamed that my guitar sound much weaker than his Squier.
In terms of build it was actually well built, and the neck had a good felling and was buttery to play although the sound was really really poor.
The Body is Poplar, and the color is Black so it's not a transparent finish, probably it uses 5 or more pieces of Poplar. Poplar is a crappy wood for guitar, and maybe I even got a bad set of poplar, even worse with being 5 or more pieces.
The Frets were worn quite fast also, so the percentage of nickel should be quite low.
The Nut was cheap plastic and broke in the first year.
The pickups I can tell if they are good or not, because the guitar itself sounds bad. I replaced the pickups at the time trying to improve it's sound and the problems maintained. It was the worst sounding strat on the block.
Saying this I think that nowadays the more expensive MIM guitars are really really good.
I have a MIM Fender FSR 72 Telecaster Custom Deluxe with Black Dove P90 pickups ($650), and this guitar is amazing. Great sound and tone. Great construction and finishing, it's at the same level as any other great guitar.