Wow... That's like asking, "How much is it to get a car?"
There's a tremendous variety in the answers you're bound to recieve. You can buy the scuba equivalent of a Kia or a Bugatti... It's really up to you. Things get even more complicated when you consider new vs. used.
For most divers, the answer truly is, "Well, you never have EVERYTHING, so it's more like an 'ongoing cost' kind of thing."
Here's even more food for thought... Some of us (like myself) do this for a living, and so the actual cost of gear is nothing as compared to the money made... So in a way, scuba gear has a negative cost for me... If I spend $100, I should make $1,000 from it. Then I get to write it off on my taxes, too.
The bottom line is that your basic C-card should cost you less than $400... Say, $350 or so. You'll need to buy your own personal gear - mask, fins and snorkel... And if you plan on doing any diving other than the stuff in a pool, you'll want a wetsuit, too, with booties for the fins. All of that stuff should run you about $400 or so... Maybe $450... But over the course of a couple of months. Basically, $1,000 in the first year can get you started.
I don't ever recommend that a new diver purchase their own scuba unit until they have a bunch of dives under their belt. Rent it all - weights, tanks, air, and all of the stuff needed is usually available for about $40 per day. I recommend doing that until you find something that you simply can't do without.
Lastly, you will find that there is no such thing as "everything" when it comes to scuba gear... They're always making some new-fangled thingamajob, and just as soon as you figure you've got everything you want, you get into underwater photography... Or spearfishing... Or figure out that you want a boat...
It's a never-ending cycle - at least, it should be.