1) From the standpoint of material costs I think you are correct, but there is a large price variation within each category so there can be some swapping of positions based on specifics. Also, labor cost can vary greatly depending on what type of frame and how intricately it's assembled. Typically carbon frames are most expensive followed by Ti. A nice steel frame would typically follow next, followed by aluminum. Steel frames vary the most in their range; from super high end to relatively utilitarian. Aluminum frames don't typically hit the high end anymore but there are exceptions.

2) As above, generalizations cloud this question. A light steel frame can be constructed, as can a heavy Ti frame. The devil's in the details.

3) A tarp would help reduce rust and corrosion on any bike, not just steel, but it's not going to prevent it. Actual climatic conditions vary from place to place and personally, I wouldn't relay on a tarp to protect any bike long term.