Typical price for combustion heater and flue removal?? $200 to $400 or $400 plus?

Would your scallion roof be radish in colour?

Uninstalled for less than 4 hunge, shouldn't be a problem.

They are simple to remove, disconnect from flue, put on trolley and wheel out the door. Flue will have a sock at roof level, you might like to get your plumber to cap this off I would ask for a quote however if it is a bog standard installation $400 should leave change. You could also have a go yourself.

cheapest way, $0, to uninstall, is to do it yourself but leave the top roof cap/vent located in the roof.
you should be able to disconnect the flue at the top inside the room and at the stove end - push the flue up and pull to the side and drop it down. There might be screws/rivets to undo where the sections are joined.

if you take out the whole flue including the section that goes through the roof the easiest way is to get a small offcut of roof sheeting the same profile you have and put this over the hole in the roof (sheet high side needs to go under sheet above it on roof for water run off).

you are probably going to have a hole in the ceiling (viewed from inside the room) no matter what you do, unless you get that patched too - not worth it I wouldn't think if you are going to put in a new one.

is the new heater flue going to have the same size diameter as the one you are removing? if so I would be inclined to leave the exiting roof cap there; otherwise rip it all out and just re-sheet the roof with an offcut.

I removed the flue and left the s/s outer sleave in the roof. I got a s/s pizza type of plate (from the $2 shop) and placed that where the flue left a hole. From the inside it looked fine and left an opportunity to re-install it all later if I wanted. Used a similar plate at the roof to weatherproof where the hole was left.