Basically, the sky is the limit, depending upon what you consider adequate and appropriate, which in turn depends on just which mode of "camping" you are contemplating.

Even within my preferred mode of camping - backpacking, costs can vary enormously. You can obtain a canteen for essentially nothing (recycled one liter pop bottle) or spend up to $82 for a titanium bottle of somewhat less capacity. An alcohol stove is also essentially free, with a trifling amount of labor, or you can spend up to about $250 for a stove which will also boil water.

Some items are key, and worth spending mucho dinero upon. For backpacking, you want to get a serviceable sleeping bag (as light as possible), a good pack, and good boots. With careful shopping, about $600 will get you all three items. Just about everything else can be accomplished on the cheap.

Then consider specialties - are you going to do any mountaineering or climbing? Good ropes run at least $150, and that is just the start. Caving? - you will need really good lights, which will not be cheap, as well as some other specialized gear.

If you are really into the outdoor scene, you will be buying, tweaking, and perfecting your gear, and you will always be spending money and/or time to make things just a little bit better. It is a lifetime pursuit.

A higher initial cost often turns out to be very cheap in the long run. That expensive down bag that keeps you warm and toasty on some grim nights and lasts for fifteen to twenty years returns its initial investment many times over. Same thing with boots and clothing, especially.

Conversely, if you don't get out and really use your gear, any money invested will be horrendously expensive.

Some excellent gear is quite cheap, and costs can be shaved considerably by judicious shopping.