As a service provider, I preferred 3 month initial commitment payable monthly with an option to continue month over month at the end of that period when a more in depth understanding of the keyword market, initial rankings achieved and services required are understood better.
I think an issue with a one time payment is that the ongoing promotion of the site is not a consideration typically. Additionally, not all are in the situation to give it a cookie cutter answer. (a) Is it a new site where seo is being provided as it is developed. (b) Is it a site that has already been developed, with little consideration to seo (c) Is it an established site (i.e. time online, links, etc) that has not been optimized (d) Is it an established site that has had seo completed previously. If (d) then what work has already been completed and the ranking/keyword situation. Each of these scenarios is a bit different, and important to consider, when purchasing services and determining costs - imo.