"Affordable" always refers to money. It means something doesn't cost too much. "Accessible" means something is within reach. It might be talking about money, in which case the two words mean pretty much that same thing. But it might be referring to other things. For example, we use "accessibility" to talk about making things easier for people with handicaps. For example, "In order to make the hotel more accessible to disabled people, they installed a new ramp at the entrance."