i get so wound up about this too, but I wouldn't begrudge paying a visiting artist/author etc either.
As a step parent to three and one of my own, all in school, it's really irritating every other week them coming home asking for money for this that or the other. And, don't even get me started on food tech in secondary school, at one point we were spending around £20 a week on food for their recipes.
All fine if you have the money, but if you are low waged, then it's a struggle, but you kind of have to grin and bear it, kids are bloody expensive.
I see it from both sides, though, i work in a school and in our department we have the most miniscule of budgets, we can hardly run our department on the amount of money that we are allocated. This year, I drew up one of those dreaded letters, asking for "voluntary contributions" towards materials used in the course. But we did stress that they are "voluntary contributions".
I felt like the enemy!! Yeah, the money we get from local government is supposed to cover this, but it doesn't. We could dumb down the projects that we do so that we use less resources... but we didn't feel that we could do that and still have a conscience although we have adjusted the projects so that we are making smaller items, therefore using less. So, we as a department thought that even if we get half of the parents to contribute a small amount, then it would enable us to buy materials and make for a better learning experience.
Unfortunately there's no easy way around this issue. Government cuts mean that this is only going to get worse.