It's been a while since I have done web development, but to me it sounds like this is a project that will take a year, and you'll never be more than 50% done be

It's been a while since I have done web development, but to me it sounds like this is a project that will take a year, and you'll never be more than 50% done because the requirements will keep changing. My recommendation would be not to agree on a fixed price for the whole thing beforehand, because there are too many uncertainties. First, come up with a rough timeline for the project. Split the project up into smaller parts. Together with your client, assign priorities to the parts (eg. payment integration: highest priority; user dashboard: lower priority). Then pick one or two of the most important subprojects, and make a detailed quote for that part. Build the first part, charge your client for it, then build the next part. After each part is built, reevaluate the timeline and plan for the next subproject. As you work on the project, priorities might change. If you do it a step at a time, you can adjust to changing circumstances.