в первую очередь can mean mainly, chiefly, but it also often implies some order, listing. While it's not explicit and necessary, it has more of an implicit undertone and gives you a freedom to continue with a list. e.g. for him profit came first, then interest. So often in a conversation you would also use a construction implying a sequence: в первую очередь нажива, потом интерес.
Hence, from the grammatical point of view, в первую очередь in the first meaning has a function of an adverbial expression (В первую очередь для него была нажива - For him profit was above all), in the second case it tends more to a parenthetical phrase (В первую очередь (во-первых), для него была важна нажива, потом (во вторую очередь) интерес).
на первом месте means more "in the first place, above all else". And besides, it is more of a stand alone phrase/construction, e.g. in the first place for him was profit. While you could continue also with giving a list in the second place, etc, you feel much less obliged to.