And yes, they are expensive! That is why when I get to my LHS I look for deals. You'ld be surprised at what some people discard or sell real cheap because a part might be missing or the handrails are broken. I recently bought a Burlington Northern F7 for $20 that had no sideframes. It had a good motor with double brass flywheels and it runs like a charm. For $5 more I ordered some side frames for it and figured an easy way to mount them. Now I have a complete loco that looks and runs great for little $$. It is also fun to build structures from simple card stock and clear plastic for windows, etc. Biggest expense are the turnouts and track but that can be done a little at a time.
So yes, some people put a lot of money into the hobby but it's not absolutely necessary to have some fun at it.