The easy answer is that the Aroma-Express is specially designed to diffuse essential oils in their most refined state for aromatherapeutic purposes. The cold-air atomizing diffusion process is also helpful when using cold-pressed oils (e.g. orange, bergamot, and other citrus oils), which lose their "punch" when heated. A candle-warmer by contrast may be useful in diffusing for scent, but it won't work for aromatherapy.
As for the cost, it is what it is. Please feel free to search elsewhere for an alternative that suits you and your lifestyle/financial needs better.
Sincerely, Autumn for the Beeyoutiful team