I assumed you were talking about the frame, since you mentioned titanium and carbon fiber. Pedals out of those exist, but they're not exactly your run-of-the-mill materials, even on higher-end bikes. Honestly, carbon pedals sounds like a shitty idea. Controls pretty much stop at aluminum or even plastic in the case of shifters, since they don't take very big forces.
That's why I talked about "parts", not "frames".
Cheap pedals and cheap brake levers? Plastic.
> The lighter materials cost more, not less
Plastic is lighter and cheaper than steel and aluminium.
Carbon is lighter than steel and aluminium but much more expensive.