Cost varies greatly. Used gear CAN be a good value or complete waste of money and it's difficult for the beginning diver to tell which it is.
At the most basic (at least around here) you need to purchase your own mask, snorkel, fins and weight belt. You may or may not use a snorkel long term (search snorkel and read other threads pro & con) so may not want to invest in a high end snorkel. That said, I bought an Aqualung Impulse and for the few times I do use it, am glad I spent the extra $$. Weight belts are pretty low cost. At the LDS where we trained you also needed 6 lbs of lead (2 X 3 vinyl coated so they don't damage the pool) and they had a decent discount if you bought the whole works from them. Masks, FIT is EVERYTHING. If it doesn't fit well it will leak and be uncomfortable. Aside from fit and personal taste, a silicone mask (not a PVC from Canadian Tire, etc) with tempered glass lens is much the same all over, but you really need to try on in person. Prices run from $50 (or less) to $100 plus. Fins start at sub $100 and go UP. Most people who buy cheap fins will soon upgrade if they do any amount of diving, so I recommend buying the better ones at the start. I have a preference for Mares Quatros, some people have had trouble with the buckle, we've three divers, 3 sets and so far have only had one fin strap break and no buckle problems.
After you're certified (if you buy it before and for some reason can't complete your course you take a sizable loss on regs, etc) and have rented a couple of different models/types of BC's & regs and are sure you're going to continue diving, then you can start buying the rest. You'll need an exposure suit (wetsuit/drysuit). Often 1/2 the rental cost is the suit as they wear quickly in the rental dept and fit is very important, so it's often the first "major" equipment purchase. I think the one piece with sleeveless jacket is warmer than the conventional john/jacket type and offers more flexibility. It's hard to buy a "bad" reg, from the mainstream companies they have goods regs, better regs and top of the line with bells and whistles regs. Same with BC's. I prefer back inflate and suggest to at least examine and if possible dive a backplate and wing before you buy. Most shops replace their rental gear on a regular basis and a decent beginners set can often be had that way. That is how I aquired my first outfit of a john/jacket wetsuit, BC & reg. All were used and showed some wear, but were perfectly functional and have had over 80 dives put on them since (some by me and some by my wife as her first gear). The reg in particular is a US Divers and well known for it's "bulletproof" design, which is why they used it in the rental dept and she still dives it. There are also a companies which have closed or been taken over for which service parts are no longer available. I've seen some great looking older Dacor regs advertised, that are of a great design, and may currently be working, but I'm told by a servicing dealer that the parts required for an annual service are no longer available, thus any money spent to acquire them is pretty much a waste. They had a limited time trade in offer for them that has since ended.
You didn't ask, but I'll also address training. DO NOT base your decision on where to train on cost of the course or how fast you can complete it. Or at least on the how fast, side, look for the slower paced class. You're taking a "life support" course, while it may not seem so, there's quite a bit to absorb and remember, IF the course is being properly run. I've run into too many divers on a dive site that shouldn't be diving. They barely remember gas laws and don't understand why they're important anyway. They don't know what the cylinder markings mean and when they get rented an out of date cylinder and can't get it filled on a trip they're shocked (then angry). It's not a particularly dangerous activity, but it does not forgive carelessness or complacency.
