it really depends on what you buy. It can be anywhere from lets say $400 for helmet/jacket/gloves (the basics in my opinion) to a few grand. I have 2-piece leathers that I picked up for $150. They were on clearance for $400 but I tried to get them to throw them in for free when I bought my current bike, we settled on $150. I have A* GP Pro gloves which are $200. I've spent probably around $1300 on 4 helmets over the course of the 5 years I've been riding plus a few hundred bucks more on a couple other jackets, a couple other pairs of gloves, plus another almost $200 for a back protector, plus $340 for boots which I also got on clearance online. so it can all add up. but its protection for your body so the more the better no matter what the cost.
so it really depends on what you want, what you can afford, how much you're comfortable compromising on quality vs cost, if you can find deals.