Well, I just checked out this Start9 and apparently the hardware starts at $500... Umbrel on the other hand, runs on a Raspberry Pi (as far as I understand), which is a lot more cheaper than that. I don't remember the exact price, but I'm almost certain it is less than $100.
Discussion
I know that price is just for the board and you have to add other stuff (storage, peripherals, etc.), but I'm willing to bet that even with those additions, it will still be cheaper. Note: I'm not computer savvy, just an enthusiast.
Start9 supports RaspberryPi. I’m running that configuration myself.
But Start9 recently rolled out their own hardware for those looking for beefier performance hence the higher hardware price point.