That question can not be answered easily, because there do not exist the necessary scientific studies.
It seams to be clear that the budget for rteaching is shifting in blended learning. So investment is necessary for technical equipment and devices inspite of investing in human ressources like teachers.
Another issue is the qualified preparation work of the course itself, the costs for the tutorial support in the distance learning phase and a lot of other investments.
An interesting study about the estimation of cost was done by Tamara Butler Battaglino, Matt Haldeman, and Eleanor Laurans in 2012. They try to estimate the costs of blended learning models and full time virtual schools as they exist in the United States.
Another article was published this week in the education week "Districts Weigh Blended Costs, Savings". Here is the entrance statement:
One of the earliest hopes for blended learning—along with the promise that it would nurture students' technology skills and foster personalized instruction—is that it would help schools and districts save money.
It would be interesting which experience you have made - please leave a comment!