Pretty much the manufacturer sets their own prices. Number and quality of employees, packaging (glass vs plastic bottles), built in drippers, purity, child-proof bottles, labeling, flavor research/development, brand loyalty, etc. The amount of nicotine can increase cost as well.
You have Mt. Baker Vapor on the inexpensive end, Halo and many other e-liquids on the high end. (pre-made e-liquid).
You can buy the USP unflavored nicotine base with the PG and VG and nicotine built in, and then add your own flavoring yourself and save a lot of money. You'll need syringes and small bottles (15 or 30 ml) to transfer the e-liquid to.
If you want to DIY your own, you can get a lot of help in the DIY area on this forum. There are even recipes for mixing flavorings from various vendors.