If a piece of bespoke software was developed by a company and the Intellectual Property was retained by the company that wrote it, but now the client of the software company wants to get that source code (and its IP) how much should it cost them? How would you calculate a fair cost for the purchase of that source and IP?

UPDATE: Just to add, the software in question is of no use to anyone else (for any legitimate purpose) as it ties in directly with the business processes of one company. It is not something that can be subsequently sub-licensed or installed outside the company in question. There are links of to third party services (but these were existing services that the bespoke software had to integrate with in the first place).