In response to the comment about keeping moisture out of the safe, electrical fires and the such, most reputable safe manufacturers ship their safes with 1 or more large dessicant bags inside the safe already. This is an inexpensive and safe way to control moisture within the safe. Furthermore, if you keep the safe inside your home, the normal humidity levels inside the house are fairly low, unless you live on the water and like your windows open.
That being said, try Costco, Sams and some of the other discount wherehouses. Check their online catalogs especially. Many of the gun safe companies have special offerings that they supply to these retailers. Oftentimes, they offer free delivery which most smaller safe dealers do not. Try to calculate the delivery charges on a 500# safe! You can get a nice Canon, Browning, Winchester etc. for far less than many other places. Good luck.