Plumbing issue

Hi all, hoping someone can guide me on an issue I’m having. In a hallway bathroom, I had a sink that was having a hard time draining but still emptying after a few minutes. I tried snaking it and using liquid plumber with no success. Decided to stop and deal with it later.

This weekend we had a guest stay over, and it would not drain at all. What’s more concerning is now in the master bathroom, the sink that is closest to that hallway bathroom is backed up unexpectedly and will not drain at all. I was able to plunge the hallway bathroom sink to get that returning to its normal slow draining. The master bathroom sink I tried plunging and air plunging for a long time. All it would do is spit back up dirty water. I took off every pipe to the wall and even snaked inside the wall but everything is clear.

Could anyone offer any additional things to try? I am trying to avoid the cost of a plumber if possible since money is tight right now. If I do need to call a plumber, what kind of cost am I looking at?