Pressure sensor without using MEMS chips
Well, Digikey has some absolute pressure sensors with SPI and I2C interfaces, going for less than $1.50 and about 40,000 in hand.Resolution is 150 Pa.

But here's the catch on homemade pressure sensors - don't do it.

Look. Just don't. You're talking about the most critical sort of application - pressure fault alarm on a ventilator. This is NOT the place for jury-rigged sensors. If you try this sort of kludge, you will open yourself to grotesque levels of legal liability. And the fact that you are not manufacturing for profit is unlikely to save you.