Depending on the basis of application, attorneys charge anywhere between $1,500 to $5,000 in attorney fee alone (not including application fees and costs) for green card. To the best of my knowledge, all fees and costs must be paid in full before any attorney would file your case, which means that although you can make the payment in installments, your application will not be filed until the full payment is satisfied.
You need to contact various qualified attorneys who can review your case and can provide you with a quotation for the legal services to be provided. You can start your search at , AILA, SUPERLAWYERS, etc.
Many lawyers would agree on a payment plan for filing for green card. Contact a lawyer in directly to determine whether or not they would accept a payment plan.
good luck!