"Forgive me, I have troubles grasping why, for the sake of a few dollars (maybe $5 or so)"
Not sure where you get the 5$ figure from?
Are you suggesting Polycarbonate in place of Alum?
Thats not a good idea, engineering wise.
Machining Polycarbonate is bad... Mmm'K
You can not machine poly without forming cracks, it's a fact that I will not even start to argue about. So you will need to use straight thread adaptors and a o-ring, which are not cheaply available right now.
If you want to use plastic right, that means a mold, which add's big $$ to your value stream.
Given how complex the pump housing is with inner pipe threads, it would be a 3-5 peice mold costing even us, with our own CNC machines, around 10-20K$ USD.
That means 20$ per 1000 units !!
Now if you only wanted to provide a unit with built in bards at a fixed size of 3/8 then the mold drops to around 5-10K, which is what we use on are indusrtial units. However you loose the wide scale system adaptablity.
Guys, I have been a design engineer for 15 years, and there are always compromises to be made, in fact design=compromise.
We will likely go to a plastic pump housing in the future, but would likely be Delrin not polycarb, and be a cost effective hybrid of injection and post machining, and be adaptable to current units
**You should know, that because of the pump action, most Al housings deal with corrosion fine, since the water action cleans excessive area's. It's different then static blocks.