This is just my opinion but unless you are doing it for a fire rating, I believe any money is better spent in concealment. Like one poster said, his buddy has a spare room in his basement and the entrance is well concealed. You could do something similar and make a void behind a painting or mirror, wall in an old closet etc etc. If you DIY then you are probably going to end up spending less and getting more in the way of theft protection.
I live in a small condo, I have a non-walk in closet that is way too wide to the point where I cannot easily get into one of the corners easily. I have been thinking about blocking off and creating a 2-3' deep void in the closet to store valuables, guns etc because I have no real good place for a safe. Total project cost is probably under $500.00 because my labor is free, you cant even begin to touch a really good full sized gun safe for that money.