I am a maths PhD researcher in a CS department, just like you describe. In a few short words, my recommendation would be that you do the PhD for its own sake, not as a tool to get a higher-paying job or to learn for an industry job. If you want the latter, you're better off elsewhere, plus you will probably be miserable in a PhD. Be sure that you get into it for the sake of doing research in a field you like (even though it surely will net you numerous other benefits) because if you don't then you will not enjoy your time and also probably be at a disadvantage compared to not doing a PhD.

Myself I'm not particularly worried about maximising income or whatever. I like what I'm studying and researching, and I'm paid enough to live a comfortable life at least for now. I could honestly do this for the rest of my life (which is what an academia career is, I suppose), but if I ever get fed up with it I also know I have loads of doors open everywhere, which is certainly a comforting thought.