My 95 Lexus ES300 timing belt broke. The cam gear is exposed. I like to know how serious is this problem? And, how much is reasonable to fix?

Very serious. If you have the time and tools you can do the repair yourself. Doing so you are looking at replacing the head Gasket, Front cover seal, Intake and exhaust Manifold seals, and Timing belt for sure. Once you get it open you want to have the Head assembly checked to make sure that no valves were bent when the timing belt broke. Depending on if you can maneuver the engine to get room to remove everything you will need to remove you may have to remove the engine to do this repair. Their are many repairs that can be performed on the motor while it is installed but there are some that can not and the only way to identify which is which you have to consider how much room you have to work and how you can manipulate the motor. I highly recommend you get the Haynes or Chilton's repair manual on your vehicle so that you have the step by step instructions on how to realign the motor and timing marks and on how to remove all the components that must be removed to do this job. One of those will also give you a list of any special tools you may need to do the job.

This is very serious as the pistons come up and hit the valves and bends them. its hard to give a price without knowing how much damage is done but i can say at least 1500.00 to 2000.00.