Coolof unless your keywords are easy then people won't (or shouldn't) be willing to offer one time fee's unless they are 50 to 500%+ over the actual price!
Why? because competition can and does change!
Month 1 could be easy and you could get in the top 10 or even top 5... then the people you bumped down are like "shit ive just lost a rank" and build more backlinks and bump you down a spot and make even more work to get you to where you were before.
SEO is a game that changes everyday. If you have any sense then you will go with someone who does monthly services and know's what they are doing (generally the price will reflect that as the people who can rank stuff actually charge higher prices because it has to be worth their while).
Anyhow if that hasn;t convinced you i don't know what will but otherwise you will be throwing your money at a wall and none of it will stick!
I do offer monthly SEO services but only ever take on a few clients.
Lewi