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.
The single most expensive component of a system built from separate components is generally the processor.
Labor costs at most computer stores are quite expensive, you are better off finding a computer savvy friend to do it for you.
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.
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.