I had a Meadowbrook cart custom made for a 17 hand percheron mare for $2000 canadian. Given the extra amount of wood, bigger wheels, special access step, height adjustment that it took to make one to fit her $1500 for a pony version sounds like a somewhat reasonable target. Depending on where you are located the Amish or Mennonites would be a good starting point. My cart was built by Mennonites near Guelph, Ontario.
Ended up ordering the Easy Entry from Pam Southward. It looks Meadowbrookish. I went about $200 over budget but I think I'll be very happy with it. Here is the site. I opted out of the extended floor but I'm getting the dashboard, rein rail and wooden slats on the armrest. Pam was really helpful.