Put money in the deal and I'm no longer doing this work selflessly, but for self-interest instead.
(I work at Formidable)
Putting money in the work makes it transactional, and that turns me off to "paid volunteer work". At that point, it's really just another label for "optional job". Volunteer work is donating your time or effort, so putting the "paid" label on it is misleading in my eyes.
I also already donate to charity so I don't see a need to get paid to donate more money, which others may have an incentive to do. This is a point to consider.
The reason why I don't like to put money in open-source work is it attaches a value on something where I would consider the work priceless and personal.
It makes the work feel cold, or transactional. I'm sure others would agree.