Heya,
$300~400 easy. To start.
You end up with a $1700 1D4, that has had over 800k actuation, which means the rest of the camera has been through a lot too. Not just the shutter. So while the shutter is likely to be replaced soon, there's other parts likely ready to be replaced too.
When for $200 more or so, you can get a used 1D4 that is not nearly as used and way past end of life.
It's just not a good buy, just because you can fix it, doesn't mean it won't just fry soon anyways in other ways other than the shutter too.
Very best,