The cost of a ResEd double (the most typical on-campus residential option) is $2010 per semester for the 2004-2005 school year. The cost of a ResEd single is $2310, and college-owned off-campus housing is $2510. Co-op doubles (the vast majority) cost $1585 per semester. A semester is 15 weeks, so a ResEd double works out to be $536 a month, including utilities like intermittent heat and sporadic Internet access. Also included are ResEd rules and Security raiding your hall every now and then. By contrast, a room in an off-campus, non-College-owned house is between $200 and $300 a month, including utilities between $220 and $400. ResEd rules and Security raids are not available in off-campus houses.
However, the typical case is for a student to either have a 10 or 12 month lease, or live in college housing for both spring and fall semester. Note that while students do not explicitly pay for the room during January, it is available to them, and when students live off-campus they do pay rent in January, even if they are not on campus. So if we assume that it's a choice between a 10 month lease or two semesters in a college-owned double (the cheapest ResEd option), then we can see that ~$4,000 for the double works out to ~$400/month, so if a student would have paid more than $400/month (including utilities) off-campus it would have been cheaper for that student to live in the double. Given the cost of heating in the 2005-2006 winter, the difference between off-campus housing and a college owned double is not too extreme. However, most upperclass-types do not live in doubles, and most off-campus rentals are much nicer than any of Oberlin's traditional dorms.
It should further be noted that for students who recieve need-based financial aid, that aid is decreased by $750 when the student leaves college-owned housing. This makes it fairly likely that the cheapest option will be a college-owned double. However, this option may be undesirable for an arbitrarily large number of reasons including: lack of privacy, odor of dorms, lack of space, lack of good kitchen, lack of personal food-storage areas, noisiness of dorms, ResEd rules and raids and inspections in dorms...