The bottom line is that your traffic will increase, and you will be able answer this question yourself. The Search Engine world is constantly changing. What worked 6 months ago, may not work today. You are going to have to invest some time for a top ranking. But the rewards are worth the time spent.
Prices vary around the Internet.
Some will charge you based on the Keywords that you are using and a per placement basis. You can't just optimize one of your home pages and have it rank the same on each Search Engine. Each Search Engine is as different as they are similar. They are constantly changing their ranking information to keep people from spamming their engines. Make sure that your Web site offers good quality content and you will stay indexed.