The cost is not the data storage, it is the satellite bandwidth required.
Yes. A 10TB RAID6 with a hot spare (so three of eight disks have to fail before you lose data) costs about $5,000. That should hold all the data from all an airline's aircraft for multiple flights before you have to overwrite them.
10TB of satellite data bandwidth costs $$$$$$$$$$$$$$$$.
And a bad guy could still turn it off.
The simple solution would be to add a simple location transmitter like the ones used for cargo containers, which could report position to the airline every few minutes from its own GPS. But a bad guy could still turn it off.