having had no experience with purchasing custom made tanks...I would be ready to pay upwards of $500 for a simple tank being hand made,and closer to a $1000 for something more involved.
Metal working is not easy,or cheap,and takes many hours to achieve smoothness that is acceptable to the craftsmen doing the job.most of them are perfectionists,and if they arent satisfied with the quality it wont leave their shopuntil it does.