There are a number of models of sustainability for OER mentioned in literature. (I) the MIT model where an institution is so big and so rich that they don't mind giving away stuff for free; (ii) the philanthropy model where funding is provided to create resources and make them available as OER; (iii) the model which makes money out of OER by providing support services such as tuition and printing; and (iv) the collaboration model where institutions co-develop curricula. There are other models but these seem the more prominent.
From an institutional perspective, the best model would be the collaborative model where multiple institutions co-develop and share courses as OER. As an example, if a program on Liberal Studies was co-developed between four universities whereby each would develop 25% of the courses and share them as OER, the course development cost for each university will be reduced by 75%. This is when the true cost savings would be seen from OER. However, unfortunately universities (at least in Asia) don't seem to collaborate much with each other as we have found out from our OERAsia survey (http://www.oerasia.org/oerasiasurvey).
Thus, if you are really thinking about going down the OER route where courses are developed by re-using OER, then it would most definitely be a good idea to get a few other collaborating institutions on board.
