Depends on what it is, what materials will be used, and whether or not you want the prototype to function or if you're using it for demo purposes only.
If you're talking about a complicated device with numerous manufactured components, it could get pricey to create a working prototype. Still, there's always ways around. Metal parts can be made on tool room equipment, rather that screw machines or cold headers or whatever. Plastic parts can be done with SLA or SLS. There are people who do rapid prototyping for stampings, etc.
