The costs from my diy build here:
T6 SWB camper build
Were:
Set of ply side panels: £90
Lining material + glue for side: £120
Ply roof panels: £75
Roof lining and glue: £45
Fixings: £40
You can see in the pictures how we did the barn doors, and the side door just had a Kiravans door store. Lining all the metal parts on these doors is more complicated, as there is no natural end point for the lining to fit to.
You haven't mentioned the floor (lino, insulation, ply etc).
We used sound deadening and thermal insulation on the sides/roof.
Pete