I have a content website, and the content is very good. I have been doing SEO on my site for four months. I have had a one-way backlink request success rate of almost 20%, and reciprocal link request success rate of almost 65%, which I think is very good.
However, I want to spend the little time I get off my university on improving my site, rather than SEO. If I hire someone, or web site design company to do SEO on my site, how much should I pay? Will adSense or something similar be able to cover the costs?
Thanks

Instead of spending figures like $500 to $1000 for SEO, look through your site. Check all the areas which you can fill with content. Content is the answer to everything with respect to SEO.

Get more and more information and use proper HTML to make those pages. Check out some SEO eBooks and concentrate on following :-

1. Links and Link Titles
2. Title
3. ALT tags for images.
4. Keyword density of a Page
5. Linking keyword to pages.
6. Heading and Large Font size Texts
7. Words on your site which have been emphasized. (Bold, Strong etc.)

does all these really affect SEO? You got any ideas where I can get any good SEO ebook? Im gonna go do some googling right away.

OpaQue suggest you a good idea or you can search videos of SEO or books about this, its not difficult, its my suggestion to
you, you can think better than me.
Thanks

Content really rocks buddy for Google & If you add quality content, the surely you will have very nice serp's
I suggest why don't u buy the seobook to start on with SEO.

You certainly need to beware of changes made to websites and how those changes might affect your standings in the search engines. I recently read about someone who used a freelance site to hire people to get her website moved to a new domain and a few other additions and she was complaining that she saw a drop in sales. When she looked at her standings she was surprised to see she was no longer on the first page of results. Well, she is back on the same freelance site ready to spend more money on the first "programmer" who says they can "fix" her website so she can have her standing back in the search engines.

To her I say, wait. Give time to see if your new domain will be picked up. Make sure you register with the search engnies but don't go spending money trying to vault back to the top of Google. It took time to get there in the first place and thus you should expect it will take time to get back there again.

Hopefully you have been keeping records and logs etc. so you can look back and compare who your competing sites were and if you see no new sites in higher standing than where your site used to be then you need to do nothing but register and wait. (assuming the freelancers you hired didn't remove or hinder previous SEO...) If however you see new domains encroaching or surpassing your ranking then you need to investigate those sites and see if you can figure out why they are higher than you. This is easy to see if your site has simply been bumped a few spots or even pages back. It becomes more difficult if you don't even know if the new domain has been registered or crawled yet!

Try to make your website SEO friendly.I suggest you look for some SEO books and then you will be able to improve your website.