Sarah Drasner offers an interesting approach to pricing contract work. The per-week increment is enough to offer some cushion, while still communicating a timescale for the work.
When I was still freelancing (or, when I was feeling more premium, doing “independent consulting”), I transitioned away from hourly pricing to project pricing after reading Hourly Billing is Nuts. For the most part, it worked well—my effective hourly rate was always higher than when I’d billed hourly, and both myself and my clients were much clearer on what we’d achieve through the work.
But timescales were the tricky bit to communicate. For certain consulting services, it sounds like per-week pricing could achieve that balance quite nicely.