Clustering is done when the cost of clustering is low versus the risk of not clustering.
I would change that slightly to Clustering is done when the cost of clustering is low versus the cost of not clustering. A risk is always a cost, but some costs are not risks. For example taking down a hypervisor for maintenance vs moving guests to another node and taking down a free node for maintenance.
Sure, using risk cost vs investment cost as cost v cost is a perfect valid way to look at it. I use that all the time in the opposite way.
You can say that risk is a cost. Or conversely, you can look at the clustering cost up front as essentially a "financial event" similar to an outage.