Jen is right, most shows don't pay and you will be shooting on spec, meaning you will go to the event and have exclusive rights to shoot the event and to market images to the riders. You will need a contract that states this up front.
I looked at your site and would highly recommend you do some serious preparation for shooting shows. This means knowing what point of stride to capture and shoot at an appropriate focal length so you are not getting photos showing distortion (big head, small rear end) because your focal length was too wide, as well as culling ruthlessly and only showing your very best work.
Go look at some of the sites of our members in the directory http://www.equinephotographers.org/directory.php
to see what they are charging to give you a ball park figure on what to charge for prints, make sure your prices aren't so cheap you will be losing money by the time you are done and undercutting the other pros out there. Shows are hard work!
When you say you will have an assistant uploading all photos to a lap top does this mean everyone will be able to see images as they come out of the camera unedited? Not a good idea in my opinion, I don't show anything until I have deleted the crap and gone through and cropped and corrected everything and upload them per rider gallery, then I contact the riders and let them know where they can view their photos.
My final advice would be don't shoot an event if you aren't well prepared and ready to provide quality, timely products and services. If you can't it will be hard to recover your reputation, the horse world is very small.
Hope that helps and make sure you sign up for a free trial as a general member, you will find a wealth of info in our archives: http://www.equinephotographers.org/general.php