Unfortunately there are usually no guarantees with SEO. A company can’t say that they will for sure be able to get you to the top for a specific keyword because they really don’t know if they can. It’s definitely not a tangible item so how do you know how much is too much? Well, when you are talking to an SEO firm I would make sure to get as much detail as possible about the campaign they are going to put together for you. Make them write everything down and say this is exactly what we are going to do for your site and this is how much it will cost and this is what we expect our results to be. for example..

1) Optimize URLs, meta info, and description tags.
2) Keyword load your site for specific keywords and terms.
3) Create landing pages
4) Get this many backlinks from these site..

There are more things that they will do but this is just an example.

Then it should say this is how long it’s going to take. Maybe 2 weeks.
it will cost you 1,200 dollars and we expect you to be ranked on the front page for these keywords!

At least now you have something in writing so they won’t be able to do nothing and take your money! “like so SEO companies will do”

If done properly for the right price optimizing your site can be a very beneficial thing!