Many times vets will allow you to make a payment arrangement with them for a certain percentate down. If you go to the vet, it may be something that can be easily treated with medicine. You wouldn't have to pay 200 to 500 unless it was something that needed major care and testing. Also if your in the US, there is something called care credit, its like a loan for pet health care. Do you have any other birds? If you do, I'd make sure you keep them separated until you know what this is.
I know when you are scared and money is an issue it may seem best to sit and wait, but trust me, that isn't the best option. If you don't go now, and it gets worse, the cost may be even greater to treat it once its progressed and you may be looking a life and death situation.
The most you'd be looking at agreeing to up front would be an office visit...You will get a lot of answers and peace of mind. If its something that is very expensive and care credit/payment arrangement isn't an option, maybe you can find someone who will adopt him for the cost of his care.