[Miata] Yay! Broke My Differential!

[Roy S. Rapoport]

Yesterday, after driving ~5 minutes and making a left turn after stopping
at a light, I started hearing a horrible metal-on-metal clanging noise.
Because of the location, I couldn't stop there, so I rolled down the hill
about 200yds and parked.  This morning I dropped off the car at PR
Motorsports (thank God for Porsche-racing friends with towing trailers
designed for low-suspension cars).  They called me later and told me I my
differential was very, very broken.  Rick's not in today, so I don't know
just how bad it is, but it sounds awefully bad.

So it looks like I'm about to get a replacement differential.  Questions:
1. How much should a remanufactured diff cost me, not accounting for Rick's
excellent time?
2. Should I consider LSD?

-roy