I agree...$400 for about 5 hours worth of work and some parts, is quite reasonable. However...
Regarding your leak... WRONG... The two bolts that attach to the transmission rear support do NOT go into the casing, but, rather thru two flanges outside on the case. So, that is NOT a source of leak. So, if they put some "felts" into the holes that is kinda ridiculous.
What is possibly leaking is next to those bolts the "snap ring" cover plate. If that is leaking, then you can:
1) support the transmission,
2)remove the rear crossmember
3) remove the cover plate and put in a new gasket...you can make the gasket yourself as its about 2 inches long and 3/4 inch wide with only the two holes in it.
Probably take about 1 hour on a rack... or jacked up high working from underneath.