Ludicrous cost in writing material available for sale (2,5)
I believe the answer is:
in stock
'available for sale' is the definition.
(I've seen this in another clue)
'ludicrous cost in writing material' is the wordplay.
'ludicrous' indicates anagramming the letters (a ludicrous spelling).
'in' indicates putting letters inside.
'writing material' becomes 'ink' (substance for writing with).
'cost' anagrammed gives 'stoc'.
'stoc' inserted inside 'ink' is 'IN STOCK'.
(Other definitions for in stock that I've seen before include "Where one may have cooked any meat" , "available" , "On the shelf" , "Held by a shop" .)