How prices work: 

Cost of Materials + Labor Involved + Demand

For plasma wraponry, most theories involve superconductors. Superconductors are EXPENCIVE. Combine that with the high price metals (Gold and Silver, possibly Platinum) for circuitry, along with a pretty hefty power supply (Superconductors need to be chilled to function, and generating plasma takes a lot of energy) you're looking in the high thousands already, and that's not counting labor.