Something like 5-7% if they sell more then 3 days worth (no commission on 1 or 2 day tickets) and it varies, base tickets will get them the least amount of commission and the Premium tickets will obviously get them more.
This was explained to my sister when she requested a quote online for tickets only for 2 days for her and her husband or something like that and she was referred to the ticket center and Disney.com. She wrote back and wanted to know why she wasn't being served and the above was what they responded with.