Are stainless steel swords cast? Is that why they're so cheap?

Fûck you, libtard åsshole.

They tend to shatter when impacting an object. 

Stainless steal is a brittle metal, for kitchen knives its suitable but not for swords  

You question proceeds from some erroneous assumptions.

First, the cost of any product is determined by market demand and availability of the raw materiel. Stainless steal has many modern uses other than making swords. Because of high demand, readily available raw materiel and high production capacity, the cost per unit is low.

Second, real swords are not cast and they are not made of stainless steel. This is because there are two types of strength used when comparing steel (as well as other materiel): tensile strength and yield strength. Tensile strength in simple terms means the maximum stress a material can take before tearing. Yield strength means how much load per unit area before the steel can no longer spring back to its original shape so it is bent permanently.

While stainless steel has high tensile strength, it also has the low yield strength which makes them inferior for making swords. This is furthermore worsen by casting because the process of casting makes the metal too hard and therefore too brittle. A cast sword if struck in combat has a very high chance of shattering.

No, they are machine formed from bar stock. They csn break or bend easier than forged steel.