One can spend a huge amount of money on sick chickens, but to be honest, and I know it sounds heartless, but I look at the flock as a whole. Keeping sick chickens is not good for a flock. It can weaken the whole flock.
I do not doctor chickens, I cull. Sometimes they have a genetic weakness in their internal organs, sometimes they ate something, sometimes they just get sick.
On a good note, though, I have had chickens off and on for years, and continuously since 2007, and have had to cull only twice due to sickness. I have over the years, found dead chickens when I opened the coop in the morning. With hatchery birds, I have found that 3-4 years is about maximum lifespan.
While it is kind of sad to lose them, it does open the space up for new chicks.
I love having the flock, keeping a flock. Individual birds in the flock come and go.
Go get it over with, the worst is the dilly dallying. Even though I sound tough, I know exactly how you are feeling.
Mrs K