A. The pressure is lower as you gain altitude because there are more molecules of oxygen and nitrogen. B. The pressure is lower as you gain altitude because there are less molecules of oxygen and nitrogen. C. The pressure is the same because there is no change in the number of molecules.
The price of a cockatiel depends on a few factors. Firstly the price depends on whether or not the bird is tamed. For a tamed cockatiel it would cost around $80 - $120 and a untamed cockatiel would cost $15-$30 (AUS) Also the colouring of the cockatiel plays a part in the cost of the bird, as more rare colours (such as lutino or pearl cinnamon) may increase the cost of the cockatiel.
If you are looking at buying from a breeder you will be looking at around 300$ for standard greys. Color variations cost much much more. Most of the money you spend on a glider are one time purchases, the cage for example. A large flight cage will most likely cost you upwards of 200$ brand new, or you can always look on websites like craigslist.