If you just need to lock it up, for a little piece of mind, you can get a lockable box for < $50. Bulldog, Hornady, Snapsafe, likely a few others you can find on Amazon like this one https://amzn.to/2FRHpoF They come in a variety of sizes, make sure it will fit whatever you need. They usually have a cable you can tie it down to something to prevent someone from just walking away with it. However, the steel is usually pretty thin, the locks are mediocre. I have one for emergency car use if I drive someplace non-permissive I can lock it away in my car for a short time. Unless I really really couldn’t afford it I wouldn’t use for home storage as a house thief will likely have enough time to defeat the cable and or lock.
The “good” handgun safes start a little over $100. The cheapest Vaultek is $140-ish (website here, I think you can also buy elsewhere), Gunvault has several models between $120 and $150 also on amazon. Barska, SentrySafe (I’m sure im forgetting a few) all have entries in this category. But somewhere around $150 you should be able to get a really decent safe, that you can bolt down or tie down, with good steel thickness, reliable locks, and maybe some biometrics if that floats your boat.
When you get into the $200+ range there is Ft Knox (as heavy-duty as the name implies), more Vaulteks, Gunbox (these don’t really look like safes) that offer more bells and whistles and in the case of Ft Knox some really robust steel.