with proper maintenance i dont see why the engine couldn't make it to 300k...

i would think it is the rest of the car that would start getting expensive.
you would eventually need to start replacing rubber things, like hoses, suspension bushing, brake lines, etc.
but i guess that would be determined in how long it takes you to get to that kind of mileage.

i dont think GM has allowed a car to be produced to last that kind of mileage with their quality control.