I know turbomolecular pumps are expensive, they have 50k RPM blades that are trying to rip themselves apart at all times. But diffusion pumps are so much simpler, why are they still so expensive? I bet if a manufacturer put their heart into it, they could make a decent $150 diffusion pump while still making 25% profit.

Is there something about their construction that I am missing?

It is very low volume market, so the production quantities are very low. That means the high cost of an eventual custom tooling allowing efficient production must be paid by the few pieces sold. Or the production involves huge amount of high skilled labor, which is expensive.
Normally with most of the consumer gadgets you are used to very cheap prices, because even when the manufacturing process is complex, the quantity of that gadget sold allows manufacturers to spend really huge money on custom tooling (and also manufacturability engineering), which allows the per unit cost to be very cheap. And because of the high volume, that tooling and engineering cost dissolves so much it becomes small part even on a cheap product.

High vacuum tech is not that trivial matter, even a very simple thing has a ton of pitfalls you can easily fall into. And the ability to navigate there is an expertize you need to pay for.
The diffusion pump may look like "just few metal plates in a tube", but who knows how they need to look like exactly and how to place them, so they really suck the residual gasses out instead of filling the chamber with the boiling oil mess...
Yes, compare to a mobile phone it is by far not that complex, so they for sure could be mass manufactured for just a bit above the rough material cost, only if there would be market to sell as many of them as there is for the phones...

I know turbomolecular pumps are expensive, they have 50k RPM blades that are trying to rip themselves apart at all times. But diffusion pumps are so much simpler, why are they still so expensive? I bet if a manufacturer put their heart into it, they could make a decent $150 diffusion pump while still making 25% profit.

Is there something about their construction that I am missing?

Nothing special. A simpified variant can even be made from pyrex with moderate glassblower skills.

BTW, small surplus turbos can be acquired relatively inexpensive, too.

Also, practice shows you don't need a diffusion or turbo pump to makes successful discharge lamps, well probably until you want to pump your own HPMs. Two-stage rotary pump in a reasonable state able to pull at least 10 microns reliably is enough, remaining impurities are easily captured by gettering action of the electrodes.

@Medved
That does make sense.

@RRK
I have seen those glass ones, but I am not a glassblower nor do I have access to any of the tools I need to do that, or else I undoubtedly would.

Turbos can be easy to find on places like Ebay, but they usually are missing their controllers or other complex electronic junk which adds cost that usually ends up being over the cost of a small diffusion pump. Usually.