Normally when I do a system for a customer, I do design sessions on an hourly rate and then come out with a fixed price quotation for the full system development.

Now this customer has thrown me a curveball: he doesn't want an hourly rate for design, either - he wants me to quote a fixed price to do all the design, too! Not that he's trying to cheap out, but he doesn't want to be in a situation where the longer design stretches out, the more he has to pay - and I can understand that.

For the business layer it was actually not too difficult to work with this, because from his original functional spec I got a good idea of what the core business objects were, and in our design agreement I defined several objects which would be covered by a fixed design price; if any new non-trivial objects were discovered, they would be considered variances, and those would be billed on an hourly rate. So far so good.

But when it comes to the UI, things start getting a lot more woolly. How many screens will there be? Don't know yet. What's going to be on each screen? Don't know yet. All we know is that it's a "dashboard" type of system, and there will be a lot of visual reporting involved e.g. gauges, graphs, etc. So maybe make it fixed price per screen design? Not a great definition; he might say that everything is going to be on one screen. Maybe a price per "visual report" design, including ability to slice & dice? Again not so easy - it might be that the entire system is just one report, and all the intelligence is going to go into how to present that segmentation.

Anyone have any ideas how to do a fixed price quotation for a UI design like this?