[osrec]

Within my company, we list tasks and assign a duration and cost to them (could be fixed, hours or days). We use this to generate a quote. Our software of choice is the projects module of https://usebx.com/app (we built this, and it's a glorified to do list, but it really works for us, even on large complicated projects).

We make it clear to the client that the quote is indicative, and may go up if their requirements/operating environment turns out to be more complex than originally described.

If they try and pin us to a fixed cost, we generally refuse the project, just because people are very bad at keeping track of what they originally asked for, even when it's in writing.