If you really want to save some money notice that two of the diaphrams will be nearly new on each 4 diaphram servo. Only replace the cracked ones.
As the inside of the door is outside of the car (rain water goes through it). The upper diaphrams weather crack while the lower ones don't. (BTW all models got rid of these exposed diaphrams around 1980).
The $66 price for complete units sounds pretty steep to me (is this place supposed to be discount?). Many of these mail order retailers are just that - retailers. Prices on aftermarket parts that approach MB list, pretty good game.
We have used the single replacement diaphrams for years but they are now only on 20 year old vehicles and we have quit stocking them.
Owner 24 bay BSC
Bosch Master, ASE master L1
26 years MB technician