Thanks!

This does not allow an estimate of the area to be covered.

Just as with a roof, the slope must be specified to get the correct area.
You need to specify how the ceiling is laid out (equal rectangles?) and what dimension has slope, and how many feet (or inches) the slope is along that dimension.

Just as an example, if the slope is 1 foot rose in a 1 foot run, the area is around 45% larger than a the floor area underneath.

What is on the ceiling now?
One option would be to cover the ceiling with plywood, than fasten the T&G to that.
It would take much more time to try and fasten T&G only to rafters.

You could also consider beadboard 'sheets' to reduce cost if that is a look you can live with.

Seriously, there are only about 15 different variables to the job that you haven't listed that will factor into the price, location and actual material chosen being #1 and #2 on the list. Get multiple local bids and don't pick the highest or lowest.