What is your methodology when you need to determine the cost of UI design if you know the cost (work hours) of programming in some project?Let's say, programming of logic in some project cost X dollars. Is there some general percentage (like 30% of programming costs) which can help determine the cost of design?The area of interest is mobile phones development (mainly business tools and 2D games, not 3D at all). There is less design work here comparing to a web design or a PC game design. 
Cost of design based on the cost of programming
programming languages;business;user interface
There is no such percentage. In the same way, there is no percentage of time you'll work on security, for example. One application must be secure and will require half of your staff to work on it. Another application doesn't have any security concerns, so the impact on your schedule will be minimal.The time required to do visual design depends on the requirements. It's not about the type of software (you quoted business tools, web applications and games).If my customer doesn't care about visual design of a website, I'll spend a few hours creating something basic, and use the remaining two months of work for other, more important aspects.If my customer cares about user interaction, I'll spend a few weeks creating carefully the different aspects of interaction design, polishing the visual aspect, etc., while caring less about the aspects which don't matter for this customer for this project.In both cases, it's still a web application.In general, you have two ways to group tasks when creating an application.The first one is sequential: it's the states you need to do in order to achieve success in your project. You start by gathering functional and non-functional requirements, you work on the specification, architecture and application design, you create tests, you write the actual code, you deploy and maintain the application.Here, depending on the level of seriousness and the scale of the project, you have the similar percentages from project to project. For a home-made tiny application, you'll always have 0% to 5% for requirements. For a large-scale enterprise application requiring exemplary QA, the time you'll actually write code will be 15% to 20%.The second one is parallel: it's the things you have to do, given that they are nearly independent from the point of view of project management and the things you may do or skip. It includes security, visual design, portability, performance, etc.Here, percentages are irrelevant. It's more a question of priorities, not the scale of the application and the required QA level. A business app may have an outstanding user experience, because the stakeholders know that the project will fail otherwise or that the UX is a competitive edge. Another business app may have nearly no design at all, because there is nothing to innovate in terms of design.