Put up flyers all over the neighborhood -- this worked for me when my cat went missing for two weeks. Calls came in a few times and came to nothing, but finally someone called, said my cat was in their backyard "right now", and I went and got him back!
My vet had already been strongly suggesting I make him an indoor cat, because he'd been beaten up regularly by other cats and was having a horrible time of it outside. He was already neutered but probably got lost from running away from another fight with the local gangster cats.
So after I got him back from going missing, I made him exclusively indoors and he took to it without a struggle, and lived out a long and happy, comfortable, threat-free life.
I seem to remember some kind of charity animal clinic in my former city, who offered reduced-fee neutering to those less able to afford it. You might want to look online for similar facilities in your area, or vets who can work out a payment plan or similar. There must be some alternative to paying full price. Similar with micro-chipping -- there can be a payment plan worked out.
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