I am a developer but only intend on doing the front-end work for getting Reddit-like upvote / downvote functionality going on an upcoming site I'm building.

I have never had to contract a developer for back-end work to implement code for me so I am quite in the dark on how much I should expect to pay and how long it could take to get the site going.

I could be taken for a ride as the developer could distort the time it would take at a seemingly regular rate (hourly/day) or could otherwise distort their rate.

Please could you give me help on this. I know you need some guidance on the nature of the site so here it is.

I have a Reddit type template with CSS and PHP included. I then downloaded Pligg code that's intended to do the job of the Reddit upvote downvote functionality.

How long would a developer roughly need to unite the theme and front end with the back-end functionality?

I do understand it's not a lot of info but I'm sure you're experienced enough to have an instinct for the size of the project.

Also, should I work on an hourly/day rate/ project payment agreement?