Without seeing a direct comparison, the things that have pretty significant effects on pricing are factors like materials (spring steel vs mild steel vs stainless steel), level of finish (mirror polish vs satin polish vs rough from the hammer), proper historical line and shaping, techniques used (hot work required or just cold hammered), construction method (raised, welded, riveted), reputation of the armorer, decoration (brass edging, engraving, etching, bluing, blackening, gilding etc), heat treatment and hardening...
For example, you can get helmet-looking object that will protect your brain with a serviceable finish in 14g mild steel, cold-worked and riveted construction for $150.
You can also get a historically shaped helmet, in a good finish, heat treated and hot raised from a single piece of 1050 spring steel, with brass edging and engraving from a well known custom armorer for $4,000