1) Affordable. That's under $500. I could afford to spend more but I don't want to spend more.
You can spend less, and achieve similar results. See #3.
2) I can move it. My son-in-law and I took mine home from Dick's Sporting Goods in the back of my station wagon. It weighs 250 lbs. and full of firearms and ammo is now over 400 lbs. Larger safes must be moved by professionals and the moving fee can add hundreds of dollars to the cost.
If you can move it easily, so can a bad guy. Anything less than 1,000 pounds should be bolted, although bolting a light weight cabinet is going to offer limited protection as the steel can tear around the bolts. In many cases, you're just wrapping up your valuables in an easy to carry off package. See #3.
3) Keeps all my firearms in one place that is safe from 8 curious grandchildren.
You could put a $20 deadbolt on a closet door and keep your grandchildren as well as most of your smash and grab criminals out. If you want to get fancy, you can obtain a solid core wood door, or a steel door, and perhaps screw some plywood inside the closet.
All of this can easily cost less than $500, offer you the same (or better) protection than a $500 "safe", and give you more storage space. It is also not portable, and will never be carried off.
4) Stronger than a metal storage cabinet. Must have a serious combination lock. Seriously thick metal all around. Three large, live action locking bolts and 2 solid steel dead bolts (on the hinge side of the door).
Don't be confused by the optical illusion. Just because it is square and steel does not make it the same as other things that are square and steel. Most $500 safes are not going to have serious combination locks, as alone, they would add over $100 to the manufacturing cost.
The metal will not be seriously thick. Steel is expensive. The last time I bought 1" plate, it cost me over $50 a square foot. Using this plate, and average size gun safe would have about $5,000 worth of steel in it.
I would give serious consideration to the reinforced closet.
If you absolutely must have a free standing unit:
A steel locking storage cabinet (like the low-end StackOns) bolted against an interior wall in a guest bedroom closet is about as good as you can get for under five hundred dollars, at least insofar as theft protection is concerned.
Consider this. You will also spend less money, and you're not giving up much in terms of protection.