Time to do some benchmarking tests: throw together a demo with, say three to five screens, the actual material used is unimportant at this stage, as long as it is representative of the kind of stuff the client will be giving you, or that you’re shooting/ have shot for this. Run a stopwatch on the time it takes to import the assets, modify them in whatever way the authoring software requires for assembly, time the actual authoring, including designing the interface, time the render duration. Time the playback testing/ integration, and don’t skimp on the testing? QC phase: accuracy IS speed, going back over and over to fix new bugs or mistakes is not efficient as killing them as you go… Now you have an approximate figure for hours spent that you can extrapolate from, and you can see where the potential bottlenecks will be, maybe even get ideas for working around them.
If your team has had experience authoring interactive DVD or BluRay, that might be similar in terms of time spent.
It all comes down to an hourly rate times hours put in.
You needed to do this at some point anyhow, so don’t bill for the testing/ benchmarking, unless it generates usable assets for the actual project.