I really like to see other folks investing their time and money in these old bikes. Makes me feel better about my '74 750K and '78 750F Super Sport. Both of my bike are riders not collector/museum pieces. $4K for exhaust (and that does not include heat shields or mounting brackets) is crazy for a bike that is only worth $4-5K in today's market.
So, my suggestion (FWIW), leave the pipes on it and enjoy the smiles per mile this ol' bike has to offer. Everywhere you ride it someone will mention "my first bike was a CB750, or when I was a kid my neighbor had one of these, or this is the bike I dreamed of having when I was a kid."
Enjoy...