Foster kitten dilemma

I've been a foster mom to stray kittens for a couple of years now.

At the moment I'm fostering a little lady called Abi. 

Abi was the only kitten in the litter we found. When I took her in she had some nasty stomach bug that caused her to drop weight very rapidly. It looked like we were going to loose her but luckily she bounced back after giving her antibiotics.

As she grew bigger I started to notice she was very clumsy. A bit unusual for a cat, but a lot of younger kittens are a bit wobbly at first. But she never got steady on her feet. It is if she doesn't have full control over her back legs.
She can't walk without falling over.
I figured she was probably ataxic, and a vet confirmed it. Somehow the signals from her brain don't travel all the way down to her limbs wich results in loss of balance and poor muscle coordination. Her back hand sways from side to side, her head wobbles and she loses balance. She also has no bite inhibition at all. When she plays she chomps down HARD and draws blood.

There could be dozens of underlying causes to her condition, and most of them can not be treated.
It -might- be an infection somewhere in her brain or nervous system, so we started her on yet another course of heavy antibiotics, pain medicine & an anti-inflammatory. If this works we should see significant improvement within a week. 
If it doesn't work we'll have some tough decisions to make.

Finding out what causes her ataxia will be very expensive, and in the end we will probably not be able to treat her anyway. 
The vet recommended putting her down if she doesn't improve. 

I really don't know what to think or what to do. 
She eats well, can use the litter box, doesn't seem to be in any pain, and she is an absolute sweetheart. Even though she can't walk properly she does her best to follow you around. While I'm writing this she is trying to climb into my lap. Seeing her struggle to walk over to me is heartbreaking.

But what is the quality of life for a kitten that can't even walk without falling over? She'll never be able to go outside and she will probably have to be closely supervised. No high scratching posts or access to staircases for her, because she will fall off and hurt herself.

Who will want to adopt a disabled kitten?
Is it fair to adopt her out when she could have a lot more underlying health problems? 
We're tight on funds as it is, so should we spend our money on a kitten that we won't be able to treat anyway while there are dozens of other kittens out there waiting for help?
I can't keep her myself because I know she will be bullied by my own cats. 

I just don't know what to do at the moment. I'm crossing fingers and toes that her medication will work and that she'll improve, but the vet warned me that it probably won't end well.
I'm spoiling her and giving her all the cuddles she wants, but I feel awful that in the end we might have to make the decision to put her down. Who am I to decide her life has to end?

I just don't know what to do.

Have you considered turning her over to an actual rescue organization (if you're not already affiliated with one, that is)? The group I foster with has funds set aside specifically for these more challenging cases--plus regular backing for general vet care for all of the other animals.

It's great that you have done as much as you have on your own; sometimes the formal non-profits have resources that are harder to come by for an individual rescuer. I know it would be tough to hand her over, but it could be her best shot at improved quality of life, and would alleviate the financial burden for you personally.

Good luck either way.

We are an actual rescue organisation. We're not in the US, so things are a bit different around here.
We run on donations and apart from discounted spay & neuter prices we don't receive any funding.

If I took her to a shelter she would be put down on the spot.

In the end, I think you are the only one that can decide the best decision for her. You are experienced with foster kittens and have had her properly vetted. There are some people who have the patience and desire to adopt special needs pets to love them and give them as good a life as they can for the time they have. It sounds as though she can eat, eliminate, ambulate, and socialize, although she will need a certain type of home, and that may be difficult to find. And the biting is a problem, which can be taxing on an adoptive family. She may not be suitable for a home with children. 

Keep us updated. You do a good thing ~ though it sounds like she was dealt a sour lot in life and you only do what you can do.

Little update:

Abi is still with me and hasn't improved.
But I just got great news... I sent in an application for her to be rehomed to a sanctuary that specializes in caring for handicapped cats, and I just received an email saying she got accepted!
It is a no-kill shelter where she will receive specialized care for the rest of her life. I'm so happy for her :)