It may cost some money and time to build a suitable one but when compared to the potential cost of a stray round I would say its a good investment. Every interaction with a LEO I've had while shooting on my property (only two) has included a check of my backstop and a friendly conversation. I could see those ending differently if you are being unsafe.
i just have a big pile of dirt i shoot into. don't use metal. rounds come back sometimes. the cheapest back stops are piles of dirt. quite effective too.
Check your local laws. Friend here in Noble county set up a nice range with table and backstop... Neighbors didn't like it and reported it, ends up in Noble county you need own something like one mile behind the backstop and it has to be clear with no roads "For Established Shooting Ranges". Funny thing is he had to take out the back stop and the table but he is free to hang a target on a temporary holder and blast away because that is just shooting in the back yard and not an established range!!! Madness
Years ago I had a dozer come in and dig a blade wide and 25 yards long 3 foot deep and pushed it into a pile. Worked great until they sold the farm next store into lots and the new neighbors built their house right behind it...
A while ago I called my local sheriff's office and asked them the same question. They said no back stop was required as long as the bullet didn't leave my property. That being said, it's better to be safe than sorry. I'd at least erect some sort of back stop just to be sure.