They are supposed to donate at the time they pick up the rags. The amount of that donation is supposed to be set by how much they can inflate the value of them. Then, they are supposed to donate whatever the householder donates toward them.
The actual cost of putting out the washtowels and asleeps is actually quite tiny. With a worldly publication, you have the added costs of paying the staff for writing up the articles and editing them. Additional cost is incurred in photos used for the magazines. Some of the journals have copyrighted studies cited, and they have the cost of seeking permission to use them. None of which affects the washtowels and asleeps (they just steal the photos they need, and have an unpaid art department put crap "art" on everything else). There is also no paid staff except those at Beth Hell that are paid a tiny stipend, and the writers are paid tiny stipends. Hence most of the expense is tiny.