Essential oils are extremely precious and should be treated with respect. They also vary in cost depending on the plant; jasmine and rose can be very costly. The lavender plant produces lots of oil, rose and jasmine produce very small amounts, and that affects the price.
Here are some tips by Margo Valentine Lazarra from her book Blissful Bathtimes (Storey, 2009):
• When purchasing essential oils, make sure the oils come in brown or blue glass bottles; the coloring protects them form decomposition caused by ultraviolet light.
• Purchase the smallest bottles in order to kept your expenses down until you discover your favorites.
• You should purchase only products marked “Pure essential oils” and check the origin of the brand.
The experienced aromatherapist can tell just by smelling the oil whether it is high-quality or has been diluted with carrier oil—such as jojoba, grapeseed, or apricot—so that it can be used in a massage or perfume blend. Quality also depends on the method of processing and extraction: how and where the plant was grown and whether the right amount of pressure and steam was used during distillation. Just keep in mind that essential oils do vary tremendously.
You might find oils or products that are listed as “essence.” These are chemically synthesized. You do not want these; although they might smell nice, they do not have medicinal properties. Inhale some essential oils sold in health food stores and pharmacies and buy those that appeal to you. Let your nose guide you.