Any OA publisher worth their salt offers a fee waiver program, and the expectation from Day 1 of OA has been that these costs should be baked into grant applications and funding requests. Many funding bodies and institutions take care of this for you, having OA funds and assistance available to take care of OA fees and to help direct publishing scientists to non-predatory publishers (Elsevier in particular can charge obscene OA fees, giving OA in general a bad name, which works to their benefit!)and also pick up the tab.