Maybe your local shelter or humain socity can give you a vet voucher or have a clinic you can take your kitten to. Because really the best way to treat any infection is to know what type it is first or you can make it worse by treating it in the wrong way or with the wrong med.
I know my vet lets me set up a payment plan when the bill is going to be really big or more than I have on that day. Sometimes you just can tell how big or small a the bill will be.
Good luck hope everything works out ok