It's a big question and a little akin to "How long is a piece of string", but.
Depends on what Camera, I have housed the 5DMK II, the Nikon D800 (Now Sold) the 1DMK IV (Current) and now the 5DMK III. I use the 1DMK IV for my Macro work, the 5DMK III for my Wide Angle, the 1DMK IV I use only the 100f/2.8 Macro Lens, so the Port arrangement for Macro is straightforward, but with the 5DMK III and wide angle, i have 3 different Ports, and a different length Port Extension for each Lens I use, 8-15f/4, 14 f/2.8, 24f/1.4, 16-35f/2.8, 15f/2.8 Fisheye.
I originally housed ny 5DMK II's with a Subal Housing, but after a couple of drowned Bodies, switched to SeaCam Housings, never had another issue.
SeaCam are pretty well the best Housing on the market for a DSLR, Nikon or Canon, but damned expensive, they are a Housing milled from a solid block of Aluminium.
By the time you buy the housing, Wide Angle front Port, Macro front Port, a Pair of SeaCam strobes, Cables, Strobe Arms, Front Port extensions depending on the Lens you have mounted & viewfinder, you can be looking at +/- US$20K before you put you Camera & Lens into the equation.
Friend of mine shoots Underwater with a Nikon D800, he Houses it in an Ikelite Housing system, much cheaper, clear Plastic type Housing, seems to work just fine and his cost is more like +/- US$5k.
My son has a G11, cost to House & a pair of Strobes runs about $2k.
If your not an avid diver, and just want to try the concept of Underwater Photography, you may well want to just look at hiring some gear, see how you like it, it's a reasonably large investment unless your looking to House say a Canon S95, then you can probably get away with a few hundred dollars, but the Key is generally light, you need strobes to light your subject otherwise everything has the blue tinge of unlit underwater Imaging.