I don’t think I can afford to sterilise my cat.

The question you should ask yourself is "Can I afford NOT to sterilise my cat?"

Your cat may have kittens and finding good homes for them will not be an easy task. If you do keep all of the kittens, the cost of food, litter and medical bills may prove to be a heavy financial burden. Also, certain costly health issues like cancer can be avoided with sterilisation.