Donald Clark has produced a very useful summary of "Estimating Costs and Time in Instructional Design". His estimates for on-line courses look reasonable. But I suggest the premium for on-line design can be reduced by omitting some of the flashy content, which does not actually help student learning.
At least for graduate students, my experience is that they are happy without the rich media integration, guest speakers and external web tools.
If you assume the author will be producing accessible materials, whatever the delivery method (as the law requires), then on-line is not such a big step.
If you dispense with Powerpoint type slides and pre-recorded "presentations" that takes a large load off the designer.
It is the interaction between students and with teachers which makes a good online course, not pretty slides and videos. A course becomes: read, try, discuss, do.
Also it is worth keeping in mind that about 40% of the cost of delivering a course is the assessment and it is how a course is assessed which students are most concerned with. Improvements made to streamlining the assessment design will pay large benefits later.