I would suspect it's much less expensive to mill down the seatpost than to mill out the seat tube.
If the seatpost gets messed up from the milling it would be easy to replace.
If the seat tube gets messed up from the milling, I suspect you would need a new frame.
I suspect a mail order 27mm seatpost would be the cheapest of all.

Good luck
PS: I seem to be a bit suspicious today.