It’s unlikely (almost impossible) for your girls to get intestinal worms from the kittens. If the kittens have fleas you might see a flea bite or two on the girls, but probably not. Each subspecies of fleas has a preferred host and a cat-loving flea will usually only bit a dog or a human if it can’t find a cat.
The one thing that is easily transferred from cats to humans is ringworm. Ringworm is not really a worm but a fungus. They have lots of different names depending on where the infection is. (Hair, groin, feet, etc.)
If the kittens have the fungus they will have little bald patches on their skin which could be red or flaky. It looks the same way on humans. Kittens aren’t born with ringworm and you would have probably seen it by now if they had it, so I wouldn’t worry about it.
Regarding the vet: $50/each is pretty reasonable but it does add up when you have multiple kittens. I would do so more calling around, including the local shelters and TNR places. You may be able to find a less expensive option or a vet that will take payments.
Vets have a hard time of it – it can cost them as much time and money to become a vet as it does to get through human medical school. They have to pay rent, insurance, taxes, employees, etc. A human without funds or insurance can go to a hospital and the government ends up covering some of the bill, but there’s nothing in place like that for a vet.
Keep looking though, even if you have to drive further than you wanted. You saved these little guys from death after their mama left. If she hadn’t left, there would have been four more feral cats in the world, having more feral babies and living short, diseased lives. Somewhere close to you there will be someone or something that can help you out since you are helping the wee ones.