You can buy a small floor safe at a home center for a few hundred dollars. Although they look small, they are very heavy and can hold plenty of your valuables. However, the size of the safe could potentially allow it to be carried away, so the safe thing is to secure it to the floor.
You can install the safe through a variety of flooring materials. Move the safe into the position you want it to rest. You will need to open the door and find the holes on the bottom of the safe. Use a marker or pencil and run it around the interior of the holes (there will be four). Once you have the holes marked, move the safe out of the way. If you have a wood subfloor you can predrill holes for the lag bolts (the instructions will suggest the correct size).
For concrete slabs, use a hammer drill and drill the holes to the correct size. Push the safe back into position and the holes in the bottom of the safe should be directly over the holes you just drilled. Now you can install the lag bolts for wood subfloors, or install the washers and nuts for the concrete anchors.