advice for sick stray

I have been helping feed a TNR stray at my apartment for about 2 years now.  He is a really nice and easy-going guy.  Recently, he has become ill.  About a week ago, he started dragging his hind legs.  I am not sure what happened to him, but he started to improve after about a day, and his appetite was great.  I brought him to the vet where they did not find any broken bones but they found a slight pleural effusion (fluid on his lungs).  To evaluate it further, they said they would need some pretty costly diagnostics like an echocardiogram and what not.  I had already spent $300 on him, and I was not sure how much more it would take to diagnose him.  Even then, the vet said it might be from lymphoma since he had never been checked for FeLV, and there would not be much to treat him with that was not really expensive.  They gave him a pain shot (Medicam), and then I took him back and let him go.

About 3 days later, it seemed like he recovered and was jumping, running, and going up stairs.  Just suddenly 2 days after his "recovery," he is back in the same state, dragging his hind legs and barely able to move.  He has shown no improvement after 2 days, and his appetite is now gone completely.  He has not eaten and drank water for at least a day.  I took him into my apartment overnight for some good rest and a dry bed, but this morning he painfully dragged himself over to the door and started yowling to be let out.  When I opened the door, he immediately dragged himself out and has just been sitting outside because I live at the top of a flight of stairs, and he cannot make it down.  I have left out a bowl of water and some dry food, and I rolled out a chair with a tablecloth over it to cover him from the rain.  He seems to be much more comfortable outside, but he yowls still on occasion and yearns to be back at his romping grounds, even though all he has been doing is hiding in some bushes.

He is such a lovely cat, and I cannot bear to see him like that.  Given my limited financial resources, I think I probably will need to put him to sleep to save him from any further distress, but I was hoping someone would have other advice.

Thanks in advance.

I'm so sorry to hear about your cat. You can get just as attached to a stray cat as to your own household cat.
Are there any low-cost spay and neuter centers where you live? You might call them for a referral to an inexpensive vet who treats stray/feral cats. See if the local SPCA would have any referrals. (I do not suggest you taking the cat to an SPCA because they would probably euthanize him.)
Also, any idea who TNRd him? If you know, contact that organization first for assistance, guidance. Some organizations microchip cats they've TNRd, some put a tatoo on them. That might help you identify the organization.
I don't know which is hurting him more--the leg or the fluid in his lungs. Of the two, I think the latter is more serious. Where any antibiotics prescribed? (Metacam is a heavy duty, ONE TIME USE ONLY [if that] pain killer.) The test for FIV/FELK is relatively inexpensive; though I agree with the vet--it wouldn't necesarily help in why he has fluid in his lungs.
Meanwhile, put some L-lysine in his wet food. Feed him the best food you can afford (Fancy Feast Classic isn't bad)
Also, in the event he pulled a muscle/tenon, sprinkle some Glucosamine in his food. You can actually buy this supplement in the grocery store, sometimes buy one get one free.
Good luck, and do keep us updated.

I will try to find a cheaper place.  He was fixed by a nice newspaper delivery lady who no longer comes to our apartment.  I'm pretty sure she brought him to the SPCA years ago.

It's fairly likely he had fluid in his lungs for a while now, as I've noticed him with mild breathing issues for the past 8 months, and the vet says that the amount of fluid is pretty minimal (I actually brought him to the vet 8 months ago for this issue, but in the interest of cost, he only did an exam, not an x-ray, and he thought he looked pretty good otherwise).  No antibiotics were given at the recent visit.

I don't think the fluid on his lungs is really the main issue, although it could be a by-product of having systemic cancer of some sort, which could cause his leg issues.

The problem with supplements to his diet is that he is eating absolutely nothing.  His favorite treats, dry food, and wet food.  Nothing at all.  Now I'm worried the Metacam caused gastritis or renal failure.

In any case, I'm at work, and I won't be able to see him for another 5 hours or so.  I'm having a neighbor check up on him.

Thanks for the explanation.
If he is literally not eating a thing, you need to syringe feed him. Have you tried baby food (meat + broth only).

I just got an update from my neighbor.  He crawled downstairs and is hiding in the bushes again.  Will not eat anything at all.  I can't stand it.  I think I'm going to go ahead and euthanize him.  He is suffering so much.

Thank you for the advice and concern.  It means a lot.

It sounds like its time and he knows who will help him.  God bless you for being that helper

Thank you so much for caring for this stray.  He knew to come to you for help.  As painful as it may be to put him down, it's more painful to watch him suffer.    You are in my thoughts.

I brought him to a second vet that thought he may have had a saddle embolus that took out his legs because they were cold and without sensation.  He was very comfortable before he went.  Thank you for your kind words everyone.

I'm sorry for your loss, but know that the cat knew he was loved, knew he was well cared for; he left this world knowing that.

I am so very sorry.  He is at peace now and went knowing that he was loved and cared for.  Thank you for caring and doing the very best for him.