My mentor is advising me to "not worry about the cost"?
Your mentor, or prospective mentor, is telling you not to worry about the cost. Presumably, they know what they are talking about and have funding that can cover the costs. A $50,000, I am assuming US, laser could be easily covered by a large NSF/NIH type grant or start up funds. If it is a piece of equipment that the mentor needs for their lab and will use in the future, then it is no big deal. You are not taking money out of anyone's pocket. That said, you should realize that you will not be able to keep the laser at the end and it will likely stay in your mentor's lab.