> But working (and billing) in weekly Sprints seems to work very well once you've convinced your client that the framework is the right way to do it. Client want a quote, but we tell them we can't really give them any sort of estimate until we see how they work (with us). So, a two week trial period is often a good way to get that going.

I like the idea of a trial period. Get them on-board with low risk to them. As this pattern of working is exactly what I want but can't get clients to do (see my reply to your sibling commenter).

A difficulty is they invariably say the last agency/project did fixed price, so do any others they're talking to, so even if your results are good why aren't you.

They don't feel safe, and it's that safety I need to give back to them.