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.