BMW 323 Forum : “What is a Differential reseal, why does it cost $550 and how long can I wait to ...”

What is a Differential reseal, why does it cost $550 and how long can I wait to get it fixed?

I spent $2500 on my car in the last 3 months ($1100 in 12/08 and $1200 yesterday). After picking up my car me mechanic told me I still have the following issues:
Front and control arm bushings torn ($450)
Alignment after bushings ($90)
Differential reseal ($550)
Oil pan gasket ($569)
Are the above prices resonable for the proposed work?