It would also double the cost for states that current hold the presidential primary at the same time as the general primary. This assumes that the national date is unacceptable for the state's general primary.
How much of the cost of holding an election are for actual election day activities, and how much are for things like voting machines, registration, etc.
There are also other costs behind the scenes, such as processing the voter lists to go to the polling places, and processing early and provisional votes. The
cost in Kane County, IL for a special election was about $1 per resident.
If I extend that to the state, a second primary election during presidential years in IL would cost between $13 M and $14 M.