It really depends on your requirements.
I use a very simple setup with a BananaPi and a 1TB disk attached. The disk is formatted with BTRFS. On the BananaPi I installed Openmediavault and then use the UrbackUp plugin.
The BananaPi is about 4 years old. Today there are much faster alternatives with ARM processor.
Total cost of the complete system is around 100-150 EURO and enough to backup a couple of laptops.
UrBackup uses very little ressources on the client. As user I do not notice when the system does a backup.
The BananaPi uses something like 3-5 Watt. So really no issue to keep it running for 24/7.