There is the cost of hosting the website and I have no idea how much that is.
But how much person-time is also required. For example would one full-time employee be sufficient in addition to all the volunteer input from moderators?
How about getting the Arduino "system" to agree to pay for the hosting and for that person but leave the rest of the management to be decided by the users. That way the important decisions about Forum features could be decided by the people who use it.