What YC is doing is bundling discounts on PCB prototyping, and 3D printed parts. The rest of the offerings are further down the stack (testing, RF, etc.) so the only thing you would be missing as a non-YC company trying to build a prototype to get into YC is discounts.
If you are hand assembling your prototypes then you only have PCB fabrication costs, which for a 2 to 4 layer board should be about $500 max for your protos (1 week turn). That should be doable for most start-ups, even if they need 3-4 spins.
What could also be valuable is having support accounts, escalation paths and relevant implementation docs/software stacks from the big CPU/Sensor/GPU/Bluetooth/Wireless companies. Getting information/support from these big companies if you have no track record can be excruciatingly inefficient.
They also are working on getting the tools that you need to design your prototype. Like Upverter (YCW2011) (https://upverter.com/) for designing your PCB.
If you are hand assembling your prototypes then you only have PCB fabrication costs, which for a 2 to 4 layer board should be about $500 max for your protos (1 week turn). That should be doable for most start-ups, even if they need 3-4 spins.