DF-15 is getting retired (if not already, or only remaining in the training brigades to clear up the remaining stockpiles).
The DF-16 and DF-17 certainly aren't cheap. This is apart from the facts where the DF-17:
1. Can actually strike father away (and potentially reaching some parts of the 2IC), and
2. Has many other more important targets to strike farther away.
In fact, I'd say that the DF-17 (and the DF-16 to a certain degree) should be reserved for more critical targets along the Ryukyu Island Chain + Taiwan, and let the TBMs + cruise missiles do the heavy work for ground-based bombardment missions.
In addition, there's the Xizang frontier to consider as well.