Could be most anything from undersized joist, to long a span, insect infestation, fungus weaking the joist, cracked joist, sinking piers.
Going to need a trained eye to figure out what needs to be done.
As far as the slab, once again it could be it sunk (very common when the wrong fill or not compacted enough) or as simple as it was not pored flat enough.
That could be fixed with some floor leveler or if it sinking by having mud jacking done.
Asking any questions about what things cost is not going to end well on this site, no one has seen the property, nows your local labor rates, material cost, has even seen the job.