You can request quotes from professional transport companies, which will probably give you the closest estimate.
Tomball to Wichita is about 600 miles, which is the same distance I shipped my horse with a transport co. And all of the quotes I received were between $400 and $700 depending on whether or not they had a driver already making a similar trip, the number of horses being transported, the size stall you want, extra equipment being shipped with the horse...etc.
It is required by all transport companies that your horse be utd on all shots and have a recent negative coggins. Some also require a general clean health check within 7-14 days of transport.
Hope this helps! :)