I'm sorry to hear about your expensive problem. Shearing a hardened crank bolt takes quite a bit of force. If the bolt was cross threaded at installation then who did the work? I'm assuming you had a timing belt done at some point unless the previous owner did. I don't know how hard it is to cross thread these bolts but what may have happened is someone used an air gun without first making sure the threads were aligned and just ran it on. An air gun does not allow for much feel. If it was cross threaded then the air gun would not have torqued it properly leading to the pin shear. Some of these bolts have been recalled for replacement as you can find here on the forum. I would also keep the bolt half you have and make sure it was the proper one, grade and type. I don't know how you can fix this for cheap. To do any work on the end of the crank will mean you have to remove it, even if you can save it. A machine shop may have an idea. I would try to find who did the last timing belt and how long ago, and if it is a professional shop you may have legal recourse.