First thing, Fenders "over-seas" really are Made in Japan Fenders, and they are highly coveted as lower cost MIA alternatives.
Cheap Fenders are Made in Mexico. Some of the things you may find on MIM Fenders vs MIA:
1. Poor setup (A nut that is hardly filed, and the strings drastically far away from the fretboard. This can be fixed easily with a setup)
2. Poor Quality control (Neck warping is a concern here. I don't think this is fixable cheaply, but not sure)
3. Pickup quality (I believe MIA pickups have polarized pickup pins as the magnets in the pickups. Where as MIM pickups have a big ole fat magnet at the bottom of the pickup, and the pins just deliver that energy. But aside from the semantics you can assume MIM pickups will sound more tinny and less refined to their american counterparts. Could swap the pickups for a $100-$200)
4. Shielding issues (These bases may have hum. It is fixable I believe. I'm not sure the cost)
5. Poorer quality hardware (The bass can go out of tune more easily, and the mechanism of tuning may be harder to control. Could swap out the tuners for $125 I think)
There are other finer details that luthiers would understand more than myself. These include Plekking, Wood Material, Finish, and coatings (the back of the neck coating can make your hand movements slower or faster).
That's all I can think of at the moment. Buying a used American made instrument is almost always a better deal than a new cheaper made instrument in my opinion.