How to make a fixed price for design sessions?

Question

Usually, when I make a system for a client, I design sessions by the hourly rate, and then go out with a fixed price for the full development of the system.

Now this client has thrown me a Curveball: he doesn’t want an hourly rate for the design — he wants me to quote a fixed price to do the whole design! Not that he was trying to buy back, but he doesn’t want to be in a situation where a long design is stretched, the more he has to pay - and I can understand it.

For the business layer, it was actually not so difficult to work with it, because from its initial functional specification I got an idea of ​​the main objects of the business, and in our design agreement I defined several objects that will be covered at a fixed estimated price; if any new non-trivial objects were discovered, they were considered abnormalities, and they would be set by the hourly rate. So far so good.

But when it comes to the user interface, everything starts to get a lot more woolly. How many screens will there be? Do not know yet. What will be on each screen? Do not know yet. All we know is that it is a dashboard system, and there will be a lot of visual reporting, for example, sensors, graphs, etc. So maybe this is a fixed price for the screen design? Not a great definition; he can say that everything will be on one screen. Maybe the price for the design of the "visual report", including the ability to cut cubes? Again, it is not so simple - maybe the whole system is just one report, and all the intelligence is going to understand how to present this segmentation.

Does anyone have any idea how to make a fixed price quote for user interface design like this?

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