could be several hundred dollars, but have it looked at now! The metal to metal is the brake pads grinding away the rotors. If this goes on too long, you could grind thru the rotor and the wheel could fall off.
Could be expensive so don't prolong. Also, it is highly unlikely that your wheel will fall off. More likely you stopping capabilities will drastically reduced. And if you find yourself in this situation, do not drive. And if you are driving experiencing this, use your emergency brake and get to a safe spot quickly. Now it depends on whether or not it is a 4X4. If it is a 4X2, it will be slightly less. If you are somewhat mechanically capable and have a repair manual it can be done for a fraction of the price:rotors (if you let them go to far so that they can't be turned-$10-$15 at a machine shop) can range between $30-$50 each and a set of new pads could range between $20-$60. These ranges depend on the quality of parts you are wanting. Also, depending on the area you are located in, pricing for shop costs and parts (parts being about 3X the costs at auto parts stores) would range $375-$650. Just some information for thought.....
The wheel will not fall off because your brakes are grinding. The area the pads contact the rotor is outside the lug bolts. It is far more likely that your brakes will fail and you will crash if you continue to drive with this condition.
bad brakes have no affect on whether or not the wheel will fall off.
Your rotor has nothing to do with your wheel falling off the worst that will happen with your brakes if you go any longer without replacing you pads is it will warp and make a really annoying screeching noise and if not replaced they will rapidly wear out your new pads