Putting hard numbers is kind of pointless. I'm going to go on a limb and assume most people reading this are either north american or european...
If you live in Montreal, you can make 75k (USD equivalent), have 4 kids, send them to college, and while its not gonna be fun by any mean, everyone will be fed, you'll be able to send them all to daycare, and in the end they'll all have the option of getting whatever degree they want.
If you live in Boston, NYC or London, make 75k, have 2 kids, you can probably make it? Their highschool may have a few more thugs than you'd like, your apartment's landlord probably won't give a damn about you..money's gonna be tight.
If you live in Tokyo. Lets just not.
Cost of living is just too different to give out blanket numbers.