My best advice is to look for business or government auctions/liquidations, where the business or facility is shutting down and everything is being sold. (Not a "going out of business sale", but the one after that, where the office furniture & fixtures are up for grabs...)
You might be able to find a "real safe". They're heavy and cumbersome, and because of that, the liquidating business is anxious to get rid of it, and other bidders/buyers don't want them. So with a little luck, you might get one for a song, as long as you can prove you can handle it and transport it yourself.
The gun safes that you regard as "too expensive" (over $400) aren't even really "safes", either by the locksmithing industry or Underwriter Laborotories. They're considered RSC's, Residential Security Containers.
So if you hunt around, you might find a large/tall safe that's actualy rated as a real safe, and is way tougher than even a retail $1500 gun (RSC) "safe". You just have to line up a few strong buddies, get a truck, and rent a piano or refridgerator dolly for a day.
The other possibility is to check all the locksmith or safe dealers in town. They may have used safes or trade-ins availible.
The two downsides are that depending on the model of safe, you may have a locksmithing bill to get the combination changed if it's not user selectable. (Only buy a safe if they can provide the combination, obviously.), and the safe will obviously not be set up for guns inside. However, making some padded plywood racks, should be pretty simple, and hopefully the locksmithing bill will still be minimal as compared to the price of an inferior consumer product.