Colleges list various costs including tuition and room and board on the common data set (google ‘[name of college] common data set’) and usually have several years of that data available. We went back several years and did the math on tuition and other cost increases for schools my kids considered. I think in the end, the figure we used for inflation was 3.5%. In reality, it varied and was often less than that, but for room and board specifically we are paying about the same for our son now as we did every year since 2014 for each of our kids (one in Chicago, two in Texas, two private, one public). Son’s was actually cheaper the last three years than the other kids have been but this year he got a 1br apartment instead of roommates and we are paying more. Our R&B cost this year is just over $15k but in the Dallas, TX area, not the northeast.
Inflation has increased over the last couple of years though, as have rents in my son’s college town, so increases may be a little harder to predict right now.