Mexican Strats go for $399 new, so that's not a bad deal (assuming we're talking $US) if it's in good condition. If possible, you should try to plug it in to an amp to make sure everything works (switches, pots, pups, etc.) before you buy.
You can go to just about any guitar board on the 'net and find pages upon pages of debate about MIM (Made in Mexico) vs. MIA (Made in America) Strats....none of which have ever reached any solid conclusion.
The bottom line is that there are many people who like the MIM's just fine (myself included).