Please get the cats spayed and vaccinated asap. You will have more kittens. Your 2 are already old enough to reproduce. Also, there are a lot of diseases that kittens cat get like feline aids and feline leukemia. That $100 should be able to both spay and vaccinate.
I have been doing pet rescue for many years. I am involved with many pet rescue groups. You said that somebody left them. Would you like me, or another rescue, to try to find them, good, loving, permanent homes where they can be indoors too?
It's a hard life for any pet to spend it's life outdoors with storms, heat, cold, noises, predators, cars, possible cruel people, or a plethora of things that make an outdoor cats life hard.
Also, please think about it soon. Adopting kittens out is always easier than adopting adult cats out.
If so, just email me. I will be happy to help you find great homes for them.
If possible, take pics and include a description. I will give you my regular email to send it to.
Good luck.