While digital inking may be cheaper in terms of production,
getting the equipment, facility and training people in using
the new equipment can be highly costly and time consuming.
Not all studios have the time and money to do this which is
why some older companies still use cels.
Toei is one of the biggest animation producers out there and
it had the power to buy a massive number of computers and 
RETAS software packages.  

As for the costs of Gengas and Dougas, it totally depends.
One thing I personally like about Gengas more than cels is 
that I'm owning the real work of an artist.
Nobuteru Yuuki never sat in a dank basement and painted
his most prized cels himself.  
I have a few video game gengas that got translated into
pixel art.  No cels existed for the drawings anyways.
Since they were by the character designer himself, they
had a great deal of value to them.