I hope that you enjoyed Saturday’s riddles. Here are my solutions to them.

1:
A RAID system.
I’m supposedly cheap (Inexpensive), but invariably more costly (as RAID systems are).
I’m one (a single system), and I’m several (multiple disks).
And after that I do for you, you tell me I’m not required after all (Redundant).

2:
The studentd daemon in macOS.
I came unexpectedly one day (in the 10.14.4 update), intended for the scholars among you (supporting Apple’s Classroom).
Those who found me saw me among their most secret things (a new certificate in their keychain).
I may have gone now (vanished after restarting), but you can be sure that one day I’ll be back (thought likely to return in the future), particularly if you join my class (Apple Classroom).

3:
4 and 5 are ‘half’ of each other, as
The Roman numeral for 4, IV, is half of the word FIVE
The Roman numeral for 5, V, is half of the Roman numeral for 4, IV.

You’re most welcome to put alternative cases, of course.