[Wender]

Olá thiago segue abaixo:

create or replace trigger TGR_PRECOCUSTO
after update on MAT
referencing new as new old as old
for each row

declare
v_cod char(8);
v_valor number;

begin

 v_cod := :new.mat_smk_cod;
 v_valor := :new.mat_prc_ult_entrada;

 update smk
   set smk_preco_custo = v_valor
   where smk_cod=v_cod;

end;
/

Testado.