They only have walk-on parts, but they increase the cost (6)
I believe the answer is:
extras
I believe this is a double definition.
'they only have walk-on parts' is the first definition.
(as in film extras)
'they increase the cost' is the second definition.
(extra costs)
'but' acts as a link.
(Other definitions for extras that I've seen before include "Add-ons" , "Bit players in film" , "Spear-carriers" , "Bit players, actors without dialogue" , "5 [BYE BYE]" .)