"According to the College Board, the average cost of tuition and fees for the 2017–2018 school year was $34,740 at private colleges, $9,970 for state residents at public colleges, and $25,620 for out-of-state residents attending public universities." - https://www.collegedata.com/cs/content/content_payarticle_tm...
So in-state is, overall, a shade more than 1/4 the cost of private.
"According to the College Board, the average cost of tuition and fees for the 2017–2018 school year was $34,740 at private colleges, $9,970 for state residents at public colleges, and $25,620 for out-of-state residents attending public universities." - https://www.collegedata.com/cs/content/content_payarticle_tm...
So in-state is, overall, a shade more than 1/4 the cost of private.