Pictures look like a ZF unit which is OEM. These can be rebuilt by places like Carquip in Boulder and California Motorsports Inc in Lake Havasu. The rebuild estimates that I recall getting were in the $600 range but you'll have to check. Guard Transmission is no longer selling the rebuild plates for the ZF units.
The following is a great ZF 915 LSD rebuild thread by Paul Guard, the founder of Guard Transmission that gives a great explanation about how these things work.
https://www.thesamba.com/vw/forum/viewtopic.php?t=619640&highlight=
I don't think the OP's prices are out of line for OEM - I wouldn't drop the price more. You could also advertise it on the early 911s registry. GLWS