First, don't assume every Fender Stratocaster is made in the United States. Throughout the history of the company they have been made in Japan and Mexico as well.
Now, the common price difference is based on where the Strat is manufactured. The US strats are commonly your more expensive guitars and you're made in Mexico (or "MIM" as they're commonly referred to) strats are generally the less expensive ones.
This price difference is a reflection of the woods that are used, the routing of the bodies, the materials used on the necks, etc...
Beyond the quality of the components, the general quality assurance in Mexico's factories are slightly more lenient than that of the Corona, CA factory. They might let a few things slip out the door in Mexico that they don't in the US. Basically, the US guitars are held to a higher standard.
With that all being said - I can say that the differences between the two guitars are pretty obvious to most cultured Strat players. But to someone just starting out, a MIM strat is a great place to start. If it's your first strat I would say that anything with the Fender name on it would be just fine. The gap in quality between the US and MIM strats has gotten a lot smaller with more mordern manufacturing techniques.
