This is probably one of the most common questions prospective divers have, but for some reason divers tend to be very unforthcoming with an answer. Granted, as others have stated, there are all sorts of factors that can come into play that will affect cost; but people who ask this question usually aren't looking for a precise dollar amount-just a ballpark figure.
Warm water: around $2000
Cold water: around $2500
Really cold water: around $4000
Can you spend less than this? Of course. If you get entry-level gear, grey market equipment, find tremendous sales, buy used gear, don't purchase a tank etc., you can certainly spend less. Can you spend more? Absolutely. Putting rebreathers aside, even with standard scuba gear you could certainly spend more if you purchased a henderson wetsuit, high-end atomic regulator, wrist-mounted computer with wireless air, dual steel tanks, spare air/pony set-up, etc. While the ranges provided above aren't going to apply to everyone, this is basically how much a person can expect to spend if they don't want to have to rent anything, and want good-quality (not entry level, not top-of-the-line), brand-new gear.