How to calculate the price for interface design?

Question

User interface

There is a task to design the interface of a web service.
There is a large TK from one text with a general description of the functionality. The output is prototypes.
How many pages will become clear only after the development of prototypes.
The hourly rate is not suitable.
Who calculates the cost of such work?

Answer 1

From the TOR you can immediately see what and how much to do.
If you can’t see it, then you don’t have a TOR

Answer 2

If it doesn’t work out in hours, then you have 2 options:
1) piecework. Determine the cost of the prototype and at the end of the work, multiply this cost by the quantity. You can preliminarily estimate this quantity itself, based on the estimate, request an advance payment, and clarify the total cost upon completion of work
2) Estimate the approximate number of prototypes and determine the total cost. Again, it can be clarified that at the end of the work, the cost can also be clarified depending on the actual number of prototypes.

Answer 3

You study technical specifications in detail, draw up an approximate web service plan and take into account the time spent on this when calculating the budget. As a result, there will be a benchmark for the number of screens plus the time spent on studying (this time would still include work).
If you are not ready to study the TOR, then either name the amount approximately, and then justify its change, or abandon the project.