"But how much does it really cost to rebuild an exact copy of a satellite you have already built once ?
What is the real cost of the loss ?"
Most Satellites are hand built one-off pieces of machinery so making another isn't much cheaper than the first. They are also a very low quantity item so companies that build them charge all the market will bear.
The real cost of losing a satellite is massive. The operating company was planning on using or leasing capability that suddenly evaporates and they will have to wait years more to loft another bird if the company can survive the loss. A competitor may get their satellite up ahead of the replacement and take a major portion of the business that the first company planned on. The first company may have contracts with penalty clauses with customers. Just getting the cost of the destroyed satellite back might not be enough.
With 2 accidents in the last two years, insurance companies might have raised their rates to insure satellites on SpaceX rockets.