>>7614151Depends on whether you want them to survive or not.
If you're OK with not, and assuming the Falcon Heavy actually comes out for the projected price, then by just squeezing bodies into an aeroshell you ought to be able to do it for $62 billion USD. This is already almost enough to bankrupt Bill Gates.
If you want to bring life support and a reentry vehicle, and/or house them in a space station so you can have all 400,000 up there at once, that's going to cost drastically more.
So your only realistic hope is to first develop an efficiently reusable launch vehicle. As long as that costs significantly less than a hundred billion dollars to develop and build, it'll still drop the price of launch enough to save you money.
I doubt you could do it as an SSTO - most likely, you're looking at a two-stage rocket, probably airlaunched for that extra bit of efficiency. Spaceplanes seem a likely choice for efficient, rapid reusability, if you do it right, but reducing the time and cost to get them back on the runway ready to fly will make or break it.
