Agree, IMO even interviews should be compensated.
Most software companies want to schedule 3-4 interviews each 2 hrs long so they should minimally compensate 1 day pay.
If we assume 15 days off that leaves roughly 245 workdays per year and with a salary of $200k that would be close to $800.
A fair amount of compensation to spend half a weekend on a project would be $400.
$100 is insultingly low for a 4 hour assignment.