I generally pay up to $100 for one as long as the case is intact, the pointer is all there (many are snapped off or short) and the face isn't pitted or rusted. I don't care if they work or not since the Faria built tachs were garbage in 1971 and have not gotten better with age. Most have dried out and fallen apart inside and are not repairable so I just convert them without testing.
Besides the main and engine harnesses you will need Midlife to create a cluster harness. The tach and non tach clusters have many wiring differences so you will need to address those.
If your car does not have the center stack (Oil, Temp, Amps) gauges you will need those since the tach has high beam and Brake system lights only.
You will also end up with two high beam indicators unless you switch to a tach speedometer with trip, in which case you will need a new lens.
It's doable, but it's not an easy swap. If it was easy everyone would do it. And I wouldn't sell non tach tachs.