In addition: currently the minimal required insurance for non-profits is
paid for by the Nationale Loterij ("national lottery") on request, so
volunteers are often insured for free (to some degree).
When I was on the board of a non-profit, that insurance requirement &
the offer by the Nationale Loterij didn't exist yet, but IIRC we paid a
couple 100 euro / year for the insurance that we had back then (even
before it was a requirement, most non-profits had insurances like that,
because it solves most likely disputes with volunteers easily for what
is a rather modest amount).