Well ... here's what I would do if I were you.....
1) Google 'hot spots on cats' ...look for images and compare. Also google 'bot worms on cats', 'wolfworms on cats' and any other possibilities the folks on here say it is. Again, look at images see what looks most like what you cat has.
2) For whatever looked most like what you cat has, google home remedies for that ailment and look for one that maybe you already have the stuff to use or is less expensive.
Another option is to look for a local animal shelter that specializes in cats, explain that you can't afford the vet and see if maybe they will look at your can and give you suggestions for care. I don't know if they will do it for you but one would think they've seen most every ailment and may be willing to help... maybe offer to donate some time helping at the shelter if they will help your kitty.