Are musketmen actually more expensive than longswordmen since the last patch or not? I can't remember.
But yeah, not only riflemen can be bulbed easily if you've more or less planned it, you can also get steel rushed much earlier than you can get gunpowder rushed. And you don't need lots of iron to rush your neighbour, 4 should be enough. If it isn't, you can always buy a couple more (it is rare that the only source of iron is holded by your direct neighbour, you just have to scout a bit to find more). And once you've got your 4-5 longswordmen, and maybe a catapult or two if you feel you really need them, you can take the iron directly from your enemies, long before the same could be done with musketmen.
You can rush gunpowder for musketeers if you want, but unless there is really no iron anywhere near you, you won't get better results (you'll faced walled, or even castled cities, you may face medieval units, which aren't notably weaker than musketeers, while classical era units are notably weaker than longswordmen).
My 2 cents, anyway.