If all you want is an "aux port," cheapest way is a tape cassette adapter. Ghetto, but it works well and the quality is about as high as your factory setup is going to get in any case.
If you want a cheap route, a new deck can be had at say Best Buy for about $80 bucks with an aux port built in. $20 for a wire harness, and $20 for a dash kit. Their install fee is $50, but you can do it yourself. We're pretty welling to help here. The only additional cost is if you're really set on that bluetooth feature. It will look fine. In fact, when you sell the car, you can always return it back to normal if you keep your stock stereo.
"We live for a good time, not a long time."
RIP Colin McRae