In my research the best robo-vacuums are $<500... and that includes a waste bin for a month's storage.

  If you add mopping to the robo-vac it doubles the price, or more, and the process to clean/swap/exchange the mopping systems is far more cumbersome than just manually mopping say 1x a week, which I do.  It takes me ten minutes to mop hard flooring in my house, but the process of constantly fixing/manipulating mops on our previous robo-vacuum was more extensive and just didn't provide good performance.