Part of how you approach the estimate is knowing how you intend or are expected to stay involved with the effort after the first pass of the work is completed. What degree of service or support do you intend to provide beyond the initial delivery during the revision or testing phases, and then again in the post-delivery phase?
The estimate for the first pass of the effort is a fixed cost. This estimate for the first pass is the time it takes to do those discreet, "clearly" defined tasks (scope) negotiated up front (statement of work). It is something you and only you know how long it might take since you were doing the work.
The subsequent passes will take the form of revisions, refinements, and late additions. This is where the customer explains that what you did is appreciated and quite nice, but not precisely what they want or need. This phase is highly variable, and so costing it out should eventually reflect that variability.
These subsequent passes can sparsely demand your time, or remain overwhelming for the near future. When the demands of a project are sparse, charging on a rate basis (by the hour, by the revision, by the additional feature complexity) makes sense for both parties involved. You can, without guilt, work on other things that may be more pressing. You may anticipate a minimum number of hours spent on this phase, and is often safe to do include some fixed period of time and cost for revision and refinement, and then move to a more variable cost after that pool of time initially set aside is exhausted.
Most efforts will involve both the initial cut and the later edits. The estimate should reflect and note the difference as it is in everyone's best interest.
- Initial Cut: 40 dedicated hours, 25 hours committed per week ~ 8 business days
- Expected Revision/Refinement: 15 dedicated hours, 3 hours per day ~ 5 business days
- Total Allocated: 55 dedicated hours, 13 business days @ base_rate
- Additional Revisions/Refinement: as needed and as available @ base_rate + some premium
Be sure to include some form of a statement of work with the cost figures (even if somewhat vague) so there isn't a big misunderstanding later about what was expected and what was delivered.