I took a look at the unit from my '87 300E and then later from my '95 E420. They are a pretty simple unit. There is a huge power transistor mounted on a heat sink with a special dielectric grease. From other posts, the grease drys out and hardens thus negating its heat dispersal qualities.
The problem I found was there is NO part number on the power transistor. Others on this forum have searched to no avail. I would hazard a guess that the price of this would be under $50 but as you know, the replacement cost of the whole unit is pushing $300 the wrong way. Good luck.
Anziani
'93 300CE 170K (almost finished with restoration)
'95 E420 117K