It's good to know why some drones are more expensive than others. (I'm going to use "drone" instead of "quadcopter" or "multicopter" since it's easier to type.)
Your basic drone will have a 3D gyro & accelerometer that will keep it flying upright. That's about it. As far as the altitude, it's up to you to constantly vary the throttle setting to keep it somewhere near the height you want. It will also drift in the wind, and you'll have to vary the directional controls to counter this. It may also rotate when pushed by the wind.
A magnetic compass can help the drone keep a constant heading. This is important, since most drones are piloted relative to the direction they are heading. (Fancier ones have additional piloting options whereby you don't need to know which way they are pointed.)
Fancier drones (like Parrot AR) have additional sensors to maintain altitude. Either a pressure sensor (barometer) and/or ultrasonic sensors aimed at the ground will allow it to accomplish this. However, the drone will still drift in the wind.
The next level up adds either GPS or an optical flow sensor (low-resolution camera pointed straight down). With one or both of these, a drone can maintain its absolute position, give or take a couple feet.
Then, of course, there's the camera. The most basic thing is a camera that records to an onboard SD card. Then there are cameras that transmit NTSC/PAL video signals over the air to a receiver (which might be combined with a screen into the controller). Or there are cameras that transmit video via wifi to your mobile device. The latter tend to have more latency (delay), making it harder to pilot (when you're flying fast).
You'll want the camera to have a relatively wide FOV (field of view). Otherwise, it's hard to know what's around the craft that you might run into.
Then there are gimbals. These keep the video stabilised. Of course, now we're in the several hundred dollar range, so perhaps that's where to stop.