i have a raspberry pi3 (4gb, 50 euros) and a pi5 (8gb, 100 euros). i run a k0s cluster (for fun) w/ pihole, plex, some monitoring and homeassistant. that for me is cost effective
You can get a 9th gen intel pc for that today in the us. Maybe not in eu ebay but in us for sure. New n100 mini pc from china companies also clock in around that total price.