Think of what you would pay for similar treatment in your own country
Private albergues are trying to make a living for the owner/operator.
Donativo places are run by volunteers, generally, and are trying to make the cost of the operation -- water, power, and consumables. With the least expensive hostel in the U.S. at $30, and the cheapest motels at $50, paying that amount would be completely inappropriate for a pilgrim who is trying to minimize cost.
Energy costs are so high in Spain that a clothes dryer now is typically 5E. Most hospitaleros will talk you into using the drying lines if the weather is good because they think the charge is obscene, but quite necessary. In that vein, showers, cooking, and heating are probably the largest expense. There is virtually no incremental cost of a bed or mat. So a family might look at its impact; if self-catering, make an allowance for the power used to cook, count the showers to be taken, and the overall lighting and heat. For some of them the cost is no greater for five than for one.
Once you have done the complex mental calculation, you will probably arrive at 5E for the first person, 4E for the second adult, and 3E for young children, all plus the cost of any meal you decide to eat. If you simplified the math, 5E each would be appropriate, the same that you would be paying at places with a set fee!! You should not feet guilty about accepting the hospitality that is being offered. They do it because they want to, and if children help out with some chores, they will earn their reduced donation. If you willingly opened your abode, what would you expect to receive? That, too, would be a good donation.
No one is resented except the obvious freeloaders out for a cheap vacation who leave nothing. If you are outfitted in $1,000 of high tech gear they will spot you!