I am very sorry for your loss--I've had a few like that too, where nothing seemed to help and you feel awful! But...
Before we get a little too hard on the vet, let me give you the flip side of the coin (as a vet tech).
VERY few vets know much about chickens. Or any bird, really, unless they have specialized in exotic species. Yep--if it's not a cat, dog, horse or cow, it's generally considered 'exotic' (trust me--I raise rabbits!). Now...here comes your new client, very worried about their chicken. You do an exam--but because she may be eggbound, you can't take her temperature safely. Trying to get a chicken to keep a thermometer under her tongue just doesn't work very well. The chicken is in very bad shape--the client has been trying various remedies rather than coming to you early on, and there's not much you can do that isn't 'major' when it comes to cost.
It's usually the way with vets, and it's not a bad way, to present the most comprehensive and most likely to help treatment plan first. In our practice, it was called the 'cadillac treatment'--the best of everything, save-at-all-costs (if possible) version. If the owner balked or couldn't afford it, you started dropping things off the estimate...for example, instead of IV fluids, you went with subcutaneous. Or orally administered electrolytes. Not as effective as IV, but more affordable and still a help to the animal...you hope!
Bear in mind that to the vet, this is something that was obviously a health issue to start with, and you spent precious time messing around with internet advice that (to the vet) was obviously not going to help. Had you come in sooner, it may have made a significant difference in the outcome--the vet isn't a miracle worker, and no, vet care isn't cheap. They too have bills to pay.
So here we are. An unsatisfied client, a deceased animal, but what has been learned?
The client (you) needs to know a bit more about chicken health care--the vets are more expensive than you're willing to put into any single chicken, so you'd best learn as much as you can so that when you really DO need a vet's help, you know it as soon as possible, which makes for the best use of your available funds.
The vet learned that not all owners of 'exotic' pets can/will afford the best treatments or even reasonable diagnostics (the xray). Encouraging the client to learn the signs/symptoms of major problems would help prevent both of you from wasting time.
What I tell rabbit people is this:
Learn how to give an effective exam to your animals, and do so regularly.
Spend the time to observe your critters often, and be very aware of slight changes which may indicate a potential problem.
It's better to act quickly than to wait, so if you don't know what the problem is *for sure*, get that vet advice so you know what you have sooner rather than later--and you'll know for next time, too.
Know what 'normal' is. Ask your local ag extension office for references to poultry people and/or your state veterinary office. Given the push to educate poultry owners on flock health, SOMEONE will be available to you with a reasonable amount of effort.
In chickens, which I'm a relative novice at, here's what *I* do when a bird looks 'off' to me:
First, spend a little time watching them. What exactly are they doing that is catching my eye? What's normal for my flock?
Catch the bird and do a full check-over. Watch the poop--if s/he poops while I'm right there, I collect it for a possible fecal exam for parasites. Check eyes, nose, beak, ears, feel the crop, feel the breathing and listen to breath sounds and heart (yep, stethoscopes are cheap--use one!). Check the vent, under the wings, and around the tail for mites, injuries, caked poop, etc.
Check hydration by skin turgor--pinch up the skin and see if it 'snaps' back easily and quickly. If not, your chicken is probably somewhat dehydrated. The longer it takes for that skin to return to normal, the more dehydrated the bird. (Another place where knowing what normal is, is important!)
Know what the chicken is and isn't doing. Eating, drinking, pooping, behaving, laying. Write it down as you note it. A flock journal can teach you a lot.
Sorry this got so long....