With safes, fire proof ones are fairly cheap (less than 500 usd).  What you need to look at are the burglary ratings - you want UL TL-15 or higher.  These are where it gets expensive, but worth it if you have a valuable collection.  Table below shows ratings and recommend values for each level of security. 

See here:  https://www.safeandvaultstore.com/pages/burglary-ratings

One thing to consider with bank boxes is to select a higher up box to protect against flood damage if you live in a flood prone area.