I can not take them to the vet because of the expense, Having sometimes more than 1000 I have to doctor my own. I rarely loose a sick one and do not have a lot of sick ones, BUT Its been a long time since I have owned a chicken close 3 years old or older---I sell them around 2 1/2 years old or a little younger and I will have young ones already laying to replace them.
The only time I have "sick" chickens is when I let them get over 3 years old. The last time I had any that old, two of them ended up with ascites. I watched the first one to see if it would get better. It didn't. The second one, we didn't let suffer. I can't afford to pay $100 or more to take a chicken to the vet when I can replace them for far less than that.
I like my chickens but they have a purpose here and it's not as pets. I could never see taking a chicken to a vet. If it's something fixable we'll certainly try to heal them. Otherwise we have to cull for the health of the flock.
I have really only had one sick bird in 10 years of chicken keeping. I have lost several to predators though. My one sick one, I culled, not because I did not want to waste time or because I have a lot of bird, but rather to protect the birds in the flock from needless exposure to a possible disease.