It would, as Prissi said, be very easy indeed to add code to allow for a monthly cost for vehicles in addition to the per kilometre cost. Pak-set maintainers could then decide whether to use monthly costs, per kilometre costs, or (most realistically) both. That would mean that this item could be added to the code without disrupting existing pak-sets, allowing pak-set maintainers the flexibility to decide which to apply.
I am considering adding this to a future version of Simutrans-Experimental. If any such change would be seriously considered for the trunk, I could also make a .patch file for that purpose, although it would not be worth the extra effort of doing so over simply adding it to the Simutrans-Experimental code unless there was a high chance of such a feature being added to the trunk. (Of course, even if I did not produce a .patch file, trunk developers could easily copy my code in Simutrans-Experimental from Github and use it in the trunk).