From an abstract point of view what is the I/O cost for a divison?

Compute rs I/O cost (forget CPU,communications, buffers)
Divison is not a primitive operation; It is implemented using a equivalent Relational Algebra expression
rs= ΠR−S(r)−ΠR−S((ΠR−S(r)s)−ΠR−S,S(r))

where r,s are relations and R,S the attributes for r,s and Π is projection

The relation r is used several times, so there is a physical image and several logical images (it could be with pointers), no need to read it twice, I guess!
How is the division operation implemented in a execution plan?