We sail, and I like to spend a few days away from home on the boat and needed my CPAP machine. I have looked at those power backups/packs and found they are expensive for what you get. In the end I went to National Battery and the folks there told me the guys who go hunting use a 100AH 12V sealed battery they sell for $100. It lasts 3 nights usually running a CPAP machine so they told me. I bought one to have it now on our boat.
When at shore it's connected to a "Battery Tender" so when we go out it's ready to go. As it turns out because I use a Resmed S8 on the boat without the humidifier my battery would last for almost a week. The S8 only draws about 1 Amp on it's own when measure even though it says 2.5 Amps on the bottom.
I know this isn't a solution for some here who want to fly, or have convenience but it's a cheaper and more durable solution for hunting, camping, boating, than using something like a C-100 at $500.
All you need is the battery and then go to an auto supply place and get the female cigarette lighter plug with either hard wire ends or alligator clips to attach to the battery and then get the male attachment for your machine. I was able to pickup the 12 Volt power supply for the Resmed S8 for $25. (for the S9 you need an inverter which is slightly more $65 I think)
So... $500 (C-100 - 2 battery system) for maybe 2 to 3 days
$100 (battery) + $3 (female end) + $25 (power supply) for 5 to 7 days depending on your machine and pressure.
It's just something to think about.