Most of the budget safes have a composite door, with a couple of 12 gauge sheetmetal "plates. The easiest way for someone to get into a safe though, is through the top or sides. If you can bolt it to the floor in a place where it limits access to the sides, then you should be okay.
I have a safe that weighs about 800 pounds, which is a little bit light. I have it bolted to the concrete in the corner of a closet. I want a bigger one, with a better security door, but I can't really afford to drop $2500 on a safe right now.
Taz:
If you want to put a safe in a mobile home, there are a couple ways you can do it. If you put a larger piece of wood under the safe to spread the load, then bolt that to the floor, you should be able to bolt the safe down on top of that.
You would have to use some decent plywood or hardwood, probably 1" thick.
You should also brace the structure from underneath, by using 2x6 lumber to tie the subfloor together. If you do both of them, the floor should be able to take a pretty hefty safe.
After that, if you are still worried about it, you could directly brace the floor underneath with a few 4x4s and cement blocks to keep them from sinking into the ground.
