OU is pretty up front with the costs, many people report they come in under the estimated costs
From their NM FAQ:
http://www.ou.edu/content/admissions/nationalmerit/nmfaq.html
Oklahoma Residents:
$4,100 for the first year
$5,800 each year thereafter
Non-Oklahoma Resident:
$6,500 for the first year
$8,300 each year thereafter