It all depends on the geometry of your part and what the requirements of your prototype are.
-If you just want something to hold and show off, and possible do some fit tests, just make an SLA or SLS printed part.
-You could machine it if it's possible with your geometry and have a nice metal part for some testing.
-There are lots of ways (some discussed above) to make cast or molded prototypes. It all depends if you plan on doing functional testing, etc.
Check out Protomold, they do quick turnaround, one-off parts for quite cheap for a variety of prototype processes.