Often there is only a tiny amount of essential oil in a plant. This oil is sold by the ml and is ve expensive. Other oils, such as orange for instance, contain much more oil and are pressed from the peels, which are leftover from the juice-industry. Those oils are much cheaper. Luckily a few drops go a long way in aromatherapy.
Oils are very concentrated substances and often work best when used sparingly.
This depends on the symptoms and is very personal, but generally I would say you should use oils that harmonize the immune-system, for instance Clary Sage, Lavender, Benzoe, Rosemary. Some are toxic, some are skin-irritants.