it's your bike. Make it into what you want.
Late model pipes like yours typically sell for less than the early trumpet style pipes. For some reason they are easier to find also.
At any rate in good condition with no major holes I would think $500-$600. Earlier pipes could easily double that, but like I said, the 77-78 style just isn't as desirable.
As far as a 4-1 goes, if you could find one and money wasn't an issue, Kerker would be the way to go. My '75 550F had a Kerker in place of the OEM 4-1 and it sounded fantastic. But since they are very rare and very expensive when not rusted out crap, Delkevic is the clear choice. About $400, stainless steel, so they never rust and weigh about half of the stock 4-4. They sound great too (although not as good as the Kerker in my opinion). My 750F has the megaphone style muffler which I prefer to the Yoshi style tube muffler.