Oh and I see you have a male and a female? That is even a bigger risk to have your cat spayed because the greater chance of your two cats mating. You can't give the male anything, so he will constantly be fighting and spraying urine around your house knowing your female is not spayed. He may attack her, he may just constantly pick on her or try to mate her. She will go crazy with this, and then once again, what if it slips your mind to take her to the vet for the shot? She will get pregnant faster then you know what is happening with a male in the house also. This is assuming that you are also not getting your male neutered. Many male cats just aren't pleasent when they aren't neutered.
Over here in Ohio you can get a cat fixed anywhere from $10-$200, it just depends on where you go. Do some research in your country and area and see if they have any low cost clinics or mobile clinics.
One last thing, you said you might not have the money to spay your cat, but say she were to get pregnant. You will then have to be ready in case she needs to be rushed to the vet if something goes wrong. All the kittens (and she could have 8) will have to be checked by the vet and you will have to pay lots for that and all their shots. Plus if the mother rejects them then you will have to bottle feed them every 2 hours and be sure they stay warm, which is a huge responsibility and weeks of sleepless nights.
While the mother is nursing, you have to keep the male apart from her because she can get pregnant again before the babies are 2 weeks old. I am sure there has to be some risk in giving a nursing mother a birth control injection, so the male will have to be seperate until the babies are done nursing, they should stay with the mother until 10-12 weeks. So 4 months of seperation. An unneutered male is a risk to a mother and her vulnerable kittens as he could kill them.
I am just pointing out the risks of not spaying your cat, not only health risks but money risks too. Basically, if you cannot afford to properly care for your pet, which included spaying and neutering and providing proper vet care then you shouldn't have a cat in the first place. Say you don't spay her and in a few years, she develops cancer or a disorder because you didn't spay her. ot only will you feel guilty because you could have easily prevented it, but now you have hundreds and hundreds of dollars to spend on vet visits and medications, maybe even surgery and your poor cat is going through all those tests and could even die!
I am not trying to sound harsh but that is the reality of the decision you have to make. On the other hand, nothing may go wrong, everything may go perfect and she may never develop cancer or get sick or get pregnant by accident. But the fact the the risk is there should give you every reason to just have her spayed. It is a routine surgery, it is no big deal, every one has it done, it is a good thing and the overpopulation of cats in this world is so out of control that why risk it?
Ok I am done, sorry to ramble.