You may want to add some more details to find a developer. The leaderboard alone has a lot of meanings. It can be a universal online leaderboard made with PHP/MySQL + AJAX or dedicated GooglePlay, AppStore, Steam, etc. leaderboard.
Each is different and requires different amount of work (and time, so money). The PHP/MySQL one might be even a one day (or few days if you need something specific) of work to be done and tested the right way.
All I'm saying is that the more details you provide here, the better chances you have to find a developer for you.