That is going to greatly depend on your competition, where you are in the rankings and where you want to be.
If you just want to be number one for your company name and this is fairly individual (e.g. Dream Garden Care, Anytown) then it is not going to be expensive.
But if your plan is to create a website about cars and become number one for the term "cars" then it is going to cost you a fortune to dislodge the well established competition - but it can be done.
One of the greatest mistakes a company at the top of the rankings makes is not to do any SEO at all. After all, why do it if you are number one? Well to put it simply other people are after your crown and so if you don't carry out periodical SEO it is an open invitation to be toppled.
I'm Tim Hill, a Search Engine Optimisation and Online Marketing specialist. I created this site to help others understand that SEO is not a mysterious black art!.
If your a newbie try the Getting Started in SEO page, otherwise feel free to dig around and learn more.