I've taught at 3 different schools..
1. A public middle school in Illinois.. I think I received approximately $2000 for about 350 kids (I think) I taught 6 classes a day on the quarter system, so I rotated through all those kids throughout the year. That budget in my opinion was adequate and I was able to buy a sufficient amount of supplies.
2. A public elementary school in WI.. I can't remember the exact amount but it was probably between $1700 and 2000? That also was a good amount for supplies.. It was a K-4 school and I had about 150 kids (again, I can't remember exact numbers).
3. Finally I am at a private school in WI and our budget is $3000 for K-8, about 180 students. This is a great budget, however it is assumed that the art supplies will occasionally be used for special projects when teachers need them (for example, paint and paper for class play scenery, construction paper for projects, etc.... of course, the classroom teachers get money for supplies, but they will use the art room supplies on occasion).
In all my jobs, I have sent out donation lists to families and gotten great responses for fabric, yarn and other odds and ends. I have used artsonia and earned approximately $300 each year to supplement our supply budget. I will ask for money for special projects or visiting artists through various funds in the school system, or ask the parent organization.
Another thing that I have done at my current school is to add one or two items to the school supply list at the beginning of the year.... so for example, all first graders might bring in 3 glue sticks for general use. I will collect all of them for community use in my classroom. It adds up if every kid brings in a little something. If for some reason some kids don't bring in the supplies, at least you have some collected supplies to start with.
I do agree that some administrators and the general public need to realize that they will get what they pay for. At a previous school I was at they decided to cancel all after school activities because a referendum did not pass. I believe the next time a referendum was up, it was passed. The community realized more money was needed to support the programs they valued. Teachers should not pay for supplies out of their own pocket. I know sometimes they feel it is the only way to provide a quality program, but how will the school and community realize that more money is needed if the teacher is producing student work with supplies they bought themselves? When you are stating your case for budget increases, I would show the prices in a catalog and explain to the admin. that with X amount of money, this is what I can buy, so I will not be able to do clay or painting with the kids.. or whatever.. Make it tangible to them. Good luck!!