your ROI answer was good - I like to say that there are many ways to measure the success of initiatives - but it depends on the objective - actually that's the change of emphasis I would encourage you to use as a way to flip the question next time its asked - creativity is not what they should be basing the ROI on - the ROI should be on all of the costs of the initiative (of which creativity is only one part of it)...if maximizing ROI is the goal, then your ROI calculation should be including 1) the investment in making sure you have the highest priority insight/unmet need and making sure you have selected accurately the highest economic value target segment 2) making sure you've selected and invested in creating the best idea (product, service, business model) to meet that need and 3) the investment in communicating in a way that connects #1 and #2 and 4) delivery, experience, operations, etc are all part of the equation.

The intention of all of these are to stack the odds in your favor that the ROI will be favorable - ROI on creativity is a trap - intended to be an easy scapegoat - which may just be masking their fear of new ideas.