Time and Material:
The time and material pricing model is best suited for projects where it is either difficult to define the scope in advance or when experimentation with different features or approaches is required. The model offers the necessary flexibility to help you retain control over project scope, even when requirements may be changing and evolving regularly.
This pricing model is also appropriate for projects with a strong research component where it is difficult to understand how long it will take until results are achieved. With a fixed cost pricing model, worst-case scenarios would need to be taken into account, which would inflate prices and have an important impact on satisfaction.
A time and material pricing model may also be appropriate for more mature projects that have moved on to the maintenance and updates stage, where modifications are often minor, but ongoing.