I would think it depends on how many of the students/families from the old school are moving to the new school, and how much the PTO has in the bank. If a third of the school is moving, and the bank has $10,000 in it, I could certainly see more than $500 being reasonable.

I don't think there's anything legal preventing one PTO from donating money to another. The question is more should they do it. In some ways it depends on circumstances, but I think it's a positive thing for an existing group to donate seed money to a new group. I'd say a healthy donation would be somewhere between $100 and $500, depending on the finances of the existing group. Anything beyond that would be too much and unfair to the existing school, in my opinion.

Yes the existing Board is leaving our school and moving to the new school as Board Members.
Should the new PTSO Board members decide or is it appropriate for them as current Board Members to vote and decide?

Can a Middle School PTO Board vote to take a percentage of funds to a new middle school they will be moving to and become PTO Board members of or do they need to request a donation from the new PTO Board that deems appropriate ? We do have a 501c at the old school. The new school isn't open yet.