How big is the difference in quality usually between an Mexican Fender and an American Standard.?

The main differences in quality are in the components used. There is not a huge drop-off in build quality from USA to MIM. Some MIM in the past used bodies and necks made in the US at Fullerton, which isn't that far away. I don't know if that still true. But the stock pickups, pots, knobs, tuners, saddles etc are all better quality on US instruments.

It comes down in the end to the individual instrument. Some USA can be a little off and some MIM can be astounding instruments. You need to try it before you buy it.

Some guy who's a professional guitarist and instructor did a great video on Youtube that you should look for. He tested USA, MIM and MIJ Stratocasters. He played the same chord progression and short solo, in each of the 5 different pickup configurations, on each instrument. You were told to guess which was which, and at the end, he told you. I guessed wrong most of the time. The USA was overall the best-sounding, but in a couple of the configurations, the MIM was equal, and close on others (and I assumed this was the MIJ). The one with the overall thin sound and inferior tonal quality, I assumed would be the MIM and it was the MIJ.

Usually about 20years

American has higher standards

The American version sounds a lot better the Mexican one is like a cheaper version

It depends on the individual instrument. MIM Fenders are great instruments, though. Here's something to consider: MIA instruments hold most of their value if you decide to sell where MIM resell for about half their original cost.

The American verison is always better made.