[SamAtt]

Can you tell us more about what the site is built on (Language/Framework)? If you've built your site on something standard like Wordpress or Django than it will be much cheaper to implement these features. If it's custom all the way things get a bit trickier.

The biggest problem with this sort of request is that a lot of people out there will say they can do it but won't do it well. Sites like scriptlance will probably get you quotes in the few hundred dollar range but their code will be of poor quality and missing important "behind the scenes" features like error handling. So you really have to be careful (and if you tell us more maybe we can help you with that)