Obviously, the price depends on the complexity and scale of the job. No two projects are the same. But in our experience, a small ad hoc project might cost £10K to £20K. A full-scale "analyse and build" project could come in at £40K to £150K, depending on its scope. If you let us know what you have in mind, we'll give you a rough idea of the likely cost.
Projects can be costed in two ways. With fixed-price projects, we agree a price to analyse your needs and define the scope of the development, and then a price for the actual work. But if you need the flexibility to change the brief as the project progresses, or to repeatedly refine certain elements (such as the look and feel of a website), we can work on a time and materials basis.
Because we work intensively with our clients to come up with the best solutions, we'll also need you to devote your own time and energy to the project, first in the analysis, then in the prototyping, and later in the testing and acceptance of the software. This is a major commitment. Your close involvement in the project at all levels and at all stages is vital to its success.