the difference btween essential oil, and fragrance oils is that essential oils are distilled from plants, flowers, etc all natural, while fragrance oils are synthetic, also essential oils have beneficial properties ie;aromatherapy, skincare,health benefits, where fragrance oils purpose is only fragrance-of course, fragrance oils come in a wider array of scents- you can find just about anything you could think of- where essential oils limited to specific plants and florals.
As far as price goes- you can pay anywhere from $3.00 to hundreds of dollars an ounce for ess. oil- depending on the particular kind your looking for- some are harder to find- and more expensive. And you can find high quality fragrance oils online for as little as $2.50 an ounce and up. You really have to search around and try different brands and suppliers to find ones you like.
as far as preservatives go you can use grapefruit seed extract- for a natural preservative in soaps and lotions and such. you can find it here(as well as good prices-selection for essential oils)
http://www.fromnaturewithlove.com
Hope that helps!