About $500 or so.
Seriously, this has been covered umpteen times here, check the search function.
Basically here they are, assuming comparable basses.
* MIA has graphite reinforced neck, MIM is not graphite reinforced.
* MIA has truss rod access at body, MIM has truss rod access at neck. I've been told that body access is better construction.
* MIA has better quality hardware, often incorporating through-body stringing. The MIM hardware is completely adequate however.
* MIA has "better" or "more expensive" pickups - HOWEVER, this is not true for the "Deluxe" basses, the MIMs are generally the same as previous year MIA or the same. Also, MIM pickups are very, very good in their own right IMO.
* MIM is usually not as good "quality" wood. Whether this matters to you is your own decision.
There are lots of models so this isn't always true. The new MIM "60's classic" series is very impressive, the best stuff coming out of the Mexico factory.
Mexican Fenders offer an extremely great value for a guitar or bass.