The sq. footage of the roof will be substantialy larger than the house area due to the overhangs, pitch, etc.....that is, IF yours is a 1 story.

Roof replacement costs vary from one region to the other. Around here, one might expect to pay somewhere between $400-$470 per "square" "square being 100 sq. ft.}, depending on whether one is using mediums {1/2 in. thick minimum at butt end} or heavies {3/4 in. thick minimum at butt end}. Other factors which can affect the cost is the pitch of the roof, whether it's required you use a class B or class C fire retardant shake, etc.

There's no "hard rule" for when a shake roof needs replacing. Generally, a medium shake roof can last about 20 years, while a heavy shake roof can last about 30 years.....but factors such as climate, shade, quality of cedar, etc. can increase or decrease the life expectancy.

A roof CAN be your "friend"...rather than "that thing you hate".