Early model 5 speed trans assembly, NIB, will run about $1300; cases only about $580. So, all gears, shafts, bearings, shift drum, etc. NIB would run about $700. Used gears and shafts only, no bearings, shift drum, shift forks, etc. might bring $400 if all in good condition and you found a buyer that needed those parts.
Rivera Primo offers a six speed "builders kit", made in the US, for about $900. If I had to replace the innards of my old five speed, I would likely go the Rivera route.:bigsmiley25: