Hi there - I'm assuming your referring to the Sigma 18-250? I have personally never heard of someone using this underwater, or anything with a similar zoom range.. so take this with a grain of salt. You "might" be able to use that at fixed focal lengths either behind a dome at 18, or flat port at longer lengths.
This is definitely not a typical underwater lens though... a more recommended starter lens would probably be the Nikon 60 mm macro or Tokina 10-17, or if you are pretty experienced, the 105 mm macro. It depends on where and what you want to shoot.
I would like around $700 for the housing, and $200 to $400, depending on the port and extension rings. The mini dome and 8" dome require specific extension rings, where the flat port can be used as is (depending on the length of the lens).