Typically, a synchronization license is issued by your publisher. However, these days there are quite a few outlets for composers (beat makers) to essentially be their own publisher. If you sell your own beat, write your own license agreement, administrate your own license records, and handle your own transaction... such as producers who sell through Soundclick or their own website... then you would be getting 100% of the cut.
However, if you have some sort of a publisher who is finding placements for your music, writing the licensing agreements, administrating the records, and handling the transaction... then they will be needing a share of that licensing fee in order to render those services. A standard music publishing deal would split the money 50/50 as cyko and pboy mentioned. But you do have alternatives. Beat Brokerz for example, is a pseudo music publisher who will take care of those aspects of the music business for you and let's you keep as much as 80% of the licensing fees.
Just remember, it takes money to make money. The good part of it is, with a music publisher, you don't have to pay that money up front. You can lean on their expertise and resources to get further in the business!