I'm paying about $190 for the sku that allows me to directly connect 4 GPUs and use an ATX PSU to power the adapter boards.

My math says that for $450 + cost of GPUs, I can get 6 cards running just by running 6 cards per system with x1 risers.
My math says that for $820 + cost of GPUs, I can get 12 cards running on one system by using two of the x4 splitters with a single system.

Tells me I'm saving $80 per 12 GPU's I want to scale.

Probably close enough to not matter, but I just think it's cool to get 12 cards working on one system. Plus, the supplier said that volume discounts are applicable.

To be honest...I could probably design a clone of their host and adapter boards pretty easily...I wouldn't have a problem having PLX sending me their silicon. If I was serious about mining, that's probably what I'd do. If I didn't have a nice office job, it's probably something that would be fun to try. It's just an over-glorified high speed inter-poser board. I'm sure PLX has a reference design that takes the guesswork out of the schematic and layout. It may be interesting to do a BOM scrub of the host/adapter boards, contact the suppliers, and see how much it costs in materials. If only I had the time. If you were semi-serious about GPU mining and wanted to fill a small warehouse, designing your own PCIe extender assembly with PLX silicon would probably be a good move given then triviality of designing the host/adapter boards.

Christian, that other x1 splitter you're linking to splits the bandwidth of a single x1 PCIe link between four cards. Given that cudaminer chokes an x1 slot with one card already, splitting that four ways sounds like a horrible idea. From what I can tell running one instance of cudaminer for a 750 Ti with 5x24 config uses about 68% of a PCIe 2.0 x1 interface bandwidth. Of course, if you find a way to severely reduce the BW required by Cudaminer, then yes, that cheaper splitter could work just fine.

The PLX based splitter that I linked two is actually an x4 to four x1 bridge. Big difference.

I'm doing all of this more just for fun than to try and make a buck. I consider all money spent on anything related to mining basically lost.