What Are My Options For A Cheap Gun Safe?

I only have a shotgun, no handguns (yet), but I want a safe place to store my firearmm, other than just using the top shelf of my closet, as my 4yr old is a climber. I of course do not keep anything loaded and the ammo is separate, but I also just recently purchased some Buckyballs, so I’d like to keep them in there also since they are not kid-friendly either.
The safes I have seen at sporting good stores are like $500! I do not have that kind of cash – what are my options? Right now I have a trigger lock on my shotgun, but I’d like something better.