Hi, I was just browsing the site and happened upon your post. As someone who has recently taken in a small stray kitten from the road, I thought I'd put in my two cents.
A friend of mine saw a kitten thrown out of a car into the street, and took her to the vet. They kept her there all day and told us later that she was unable to use her hind leg, which might heal or might need to be amputated, and that a few days would tell. Since my friend has 3 dogs I took the kitten home and kept her in a box in the bathtub. She recovered the use of her leg and is doing extremely well now. This is, of course, after a lot of work and vet bills and almost 7 weeks.
The vet decided not to charge my friend for the first visit, calling it a good samaritan case. Maybe you can find a vet who would be willing to look at your kitten for free on this principle.
Our kitten was 4 weeks old when we got her, and covered in fleas. I bathed her in Dawn dishwashing soap, and it worked pretty well, at least until she was old enough to use Revolution (the vet also gave me a free sample of this when she was old enough, 8 weeks).
I am also of the opinion that if you can't afford to take care of the kitten, find someone who can. I agree that it's probably best to take the kitten to the vet, for his health's sake. The first vet visit is not the least of your financial problems. He will need shots and checkups regularly, as well as flea medication and exams for any other problems that might pop up especially with a stray (all the hair on the tip of our kitten's tail fell off one day, which merited a $70 vet trip), not to mention food.
Also, before I had decided to keep the kitten, my friend sent out an email to our department at work, asking if anyone had a home for it. Someone wrote back saying they would be willing to contribute to the financial costs, so maybe that is an option for you if you could talk to some kind souls, especially if you do not plan to keep the kitten permanently.
All that being said, I think it is so wonderful of you to take the kitten in and care for it, and that's what needs to be done. You will feel a lot worse if he gets worse because you haven't done all you could, and the vet is really the only one who would be able to tell you what needs to be done (especially with an infection). In my case I was lucky and our kitten did begin to recover the use of her leg after the vet's suggested few days in our care, I wish you the same luck if that's what you decide. It's a terribly hard decision to make, whether to bring it to a shelter or not. All I can say is that maybe the shelter would be a good option if you're not able to keep it, they may be able to give it care and medication, and you should not feel bad if you're absolutely not able to keep it because if nothing else you've helped the kitten from suffering alone in the street. It's such a shame and it really burns me up that people are not willing to spay/neuter their cats to prevent stray kittens like these. Sorry for the long post! Good luck and thanks for doing what you can to help.