You can get brand new Honda pipes for about 225 to 250 per pipe (HM341s) - not including shields, connectors and diffusers - aka baffles. Hondanuts will ship 4 pipes for $55 from NY to CO.
I bought a new K2-K6 seat from them and I was happy:
http://www.hondanuts.com/cb750.html18300-300-047 Muffler #1 $250.00 K0-K1
18300-341-907 Muffler #1 $225.00 K2-'76
18311-300-320 Diffuser $30.00 K0-K1
18311-341-900 Diffuser $25.00 K2-'76
18315-300-020 Muffler Protector $55.00 K0-'76
18350-300-037 Muffler #2 $250.00 K0-K1
18350-341-907 Muffler #2 $225.00 K2-'76
18400-300-037 Muffler #3 $250.00 K0-K1
18400-341-907 Muffler #3 $225.00 K2-'76
18450-300-047 Muffler #4 $250.00 K0-K1
18450-341-907 Muffler #4 $250.00 K2-'76
18472-300-000 Tube Muffler Connect $10.00 K0-'76
18473-300-000 Clamp Muffler Tube $7.00 K0-'76