Depends upon the configuration
The CPU, obviously.
Replacing the CPU is safe if you unplug your computer first.
Replacing a CPU does require a static free envioroment,and some technical knowledge.
It is more expensive than a single CPU socket motherboard
A laptop if you want a complete computer that you can carry around (there is a CPU in a laptop)A CPU if you have a computer and want to upgrade or replace the "Central Processing Unit" which is the most expensive and complicated component.
Yes, replacing a starter solenoid is expensive. It is about $80.00 for the actual starter plus any man hours that go into installing the starter. It will end up costing around $400.00.
Labor costs at most computer stores are quite expensive, you are better off finding a computer savvy friend to do it for you.
The single most expensive component of a system built from separate components is generally the processor.
SRAM, what the cache on your CPU is.
Most of the do. But there are some expensive models which have it's own processor.
qpi is a new type of bus for cpu's, replacing the traditional front side bus. it stands for quick path interconnect
BIOS of what component? your graphics card will have a BIOS, the CPU will have a BIOS. If there is nothing wrong with yours don't bother replacing it.