If you are concerned about the housing price, you need to realize that you have not even got started yet. Add a couple of strobes, some ports, some more lens, a focus light, a video light, a tray, and an arm set to your costs. Take your camera cost and multiply by 10. Then be happy if you get in the water for less.
That said, there are way cheaper underwater setups, but not of the caliber of your camera.
If you are just starting out with under water photography, I suggest you shop for a very simple used setup. Maybe a P&S with just a housing.
Try it out and use it to learn what you really want and will need. Do you like macro, or are you just a wide angle freak? The type of pictures you want to take will dictate some of your equipment.
Consider it a disposable learning cost. There is a lot to learn about the equipment and what things are compatible. I too often see posters asking how to make their new strobe work with their camera and the answer is either a $200 cable or a $600 converter of some sort.
As an example, I was able to obtain a used 10mp cannon powershot and case from a little old lady who had used it for snorkeling for only $200. It is small, compact, fits in my BC pocket and is now used to capture "happy snaps" by my dive buddy. It is capable of taking some excellent shots, but only under a set of fairly limited conditions. It is a great starter setup and the cost is extremely small in comparison to a full blown dSLR rig.
