If you can address the sagging from beneath, you will have the option to redo the floors, which is most certainly your cheapest option. But sometimes sagging is best handled by ripping up the floors and sistering the underlying joists, which would require floor replacement.
If you have the option to redo the floors, then most flooring contractors can do a good job of it, and even match existing flooring in adjacent areas, if needed. But if the flooring itself is in disrepair (bad planks, wide gaps between planks, deep water or pet urine stains, holes, etc), usually it's best to just start with fresh flooring.
500sft isn't a huge area to replace. Since this is a rental, you may want to look at luxury vinyl plank or better yet Floor and Decor has a product called Nucore, which is at ~$2.75/sqft. is about the same cost as good LVP, looks great, and is nearly indestructible... that would actually be my first choice.
https://www.flooranddecor.com/nucore-flooring