Client wants to buy the IP for their new app AMA

We produce Enterprise iPad Apps - I have a client who wants the option to purchase the IP to an app we are developing for them. We produce an Enterprise App that is customised for each client we take on. This customer wants to enhance our normal product and have the rights to the IP. 1. So no one else can have what they have. 2. So that a possible future joint venture between us may be formed so that other people in their industry may have the app that they hold the IP for- they'd like to set up a licensing deal for this. We haven't done this previously. I'm looking for any advice as to how to price for what they want, i.e. That is how much uplift over a standard-semi custom app might be expected. Or to put it another way does anyone have any experience of selling a variant of their standard product to a client who would then hold the IP to this version? I appreciate this is a pretty complex question. I'm mearly looking for input to consider