I go to extreme measures to make my pens predator-proof, and have not had anything breach my defenses in years (the early years were when I learned how to protect my birds - by experiencing some losses). I do this with these rules:
1. cover everything - raptors fly and coons climb.
2 Add a full 2 ft of "apron" around the coops and runs. This is just 2 ft wide galvanized woven wire laid flat on the ground, with weights to temporarily hold down the wire until the grass grows through it. It's invisible and very effective in stopping all diggers.
3. Good, string doors - well secured and often padlocked.
None of this is very expensive really. It does add some costs and a lot of time to get it right, but I could not stand to have my flock decimated by predators as Stake has had to deal with. It is so worth it to have them secure, I sleep well because of it.