Can I give my cat amoxicillin?? If so what dose?

NO! You can't just give him regular medicine because it is likely you will get the dosage wrong or miss something else. If cost is an issue (which is totally understandable) try seeing if there is a vet clinic/or vet school in your area (both tend to be much cheaper then traditional vets) if not, just call your vet and have an honest conversation saying something like" here is what is happening w/ my cat, here is what I can pay, what are my options?"-he/she should either help, or refer you to someone who will--an overdose of "human medicine" could kill your cat, so just shop around a bit and see what you find.