Source? Every company I've heard talk about their reasons for open plan trumpets collaboration, not cost.
Someone pointed out you do get to know your coworkers better, but I'd argue you just get to know them better, faster.
Cornell did a study that showed that people interact less when they have offices. Their definition of "frequent interaction" dropped from several times a day ad hoc (with open plan) to several times a week in a meeting. That's some data supporting that it does in fact improve collaboration.
Archive.org is down right now but I can supply a link to the study later if you want.