Easiest/cheapest way to play music on a 2005 BMW 645ci with my Iphone?

So I just recently bought a 2005 BMW 645ci. I'm not too familiar with BMW's but I've been searching online on how to connect my Iphone to play music in the car. The first way I saw was using a USB/AUX port (which I don't have in my car). Second way I saw was using Idrive to connect to bluetooth and from there you can play audio through the Iphone (that won't work for me because I have an older version of Idrive that doesn't have the same options as the tutorial videos that I found on youtube). I was thinking either updating my Idrive by going to a BMW dealership (don't really know how much it will cost). But other than that, I'm out of ideas… If you can please help that would be great!

Added (1). I do have bluetooth, but all it syncs is my contacts and phone calls, it doesn't have the option for syncing music.

Your car doesn't have blue tooth? My 325xi has blue tooth. You can just connect through blue tooth and play without it being plugged in.

Should have bluetooth. If not, you'll need an FM transmitter or new stereo.

There's no Bluetooth device in your car. So you can't play song in your car without any external Bluetooth device. You may play song by fm.