To reiterate the previous replies: If you are an employee, they can not back charge you. They can cut pay, fire, mandate more training, require retesting, pull your certification but you can not be financially responsible. It is the contractors financial responsibility.
If you are the sub contractor then you are responsible for the work that you performed. Normally you will be allowed to repair your own work. If they are paying for the x-rays then they could charge you for the additional x-rays. Those matters should be adressed in a preconstruction meeting (before work is performed) if you are a sub. So should the quality requirements that you must comply with. ASME IX welder qualification test can be examined by x-ray or by guided bend test. Either way you are qualified to make x-ray welds in the field if your qualification covers the process, pipe size, etc.
If you are an independent contractor it's always a good idea to know what, if any NDE will be performed and what the product (pressure and temperature) is. It helps to know beforehand all of the factors that you are dealing with.
As you said, live and learn.
Good luck to you.