That all depends on your goals...
Some companies charge 500 a month... some 1000 a month.
Be carful....
If anyone says they "Guarantee" 1st page you should probably steer clear of them.
If anyone says there is only one way to rank a site, steer clear of them as well.
What are your expectations ?
What results are you wanting ?
What is your time frame for achieving results, 6 months, 9 months, or a year ?
Many questions that need to be adressed....