Fumed silica and silica fumed are two different terms for two substance. They are both silica dioxide. On some occasions people confused with them. They have different derivations, technical characteristics, and applications, then the price is different.
Fumed silica, also known as pyrogenic silica because it is produced in a flame, consists of microscopic droplets of amorphous silica fused into branched, chainlike, three-dimensional secondary particles which then agglomerate into tertiary particles. The resulting powder has an extremely low bulk density and high surface area. Its three-dimensional structure results in viscosity-increasing, thixotropic behavior when used as a thickener or reinforcing filler. -Via wikipedia
Fumed silica normally classified into two classes: treated and untreated(also known as hydrophilic and hydrophobic). Normally the hydrophilic one is cheaper than hydrophobic.
The particle size of fumed silica is around 7~40 nm. The most commonly used unit is BET surface area(m2/g), a 40′ HQ container can load 3.6~4 Mt fumed silica inside. Fumed silica require very strictly moisture-proof method: packaged in kraft paper and rounded by plastic layers.
Silica fume, also known as microsilica, is a fine-grain, thin, and very high surface area silica. -Via wikipedia
The particle size of silica fume is around 1/100th the size of an average cement particle. The most commonly used unit is mesh. The main application is concrete.
The more information of silica fume can be found here: What is fumed silica