I was the parent volunteer who collected for the teacher holiday gift a few times. Most parents donated $5-$10. A few gave nothing; a few gave $20; and a few told me that they preferred to give their own gift. We combined the gifts into one Visa gift card.
Some years no class gift was collected. In those years, I had my kid make a card and we put a $10 Target gift card inside. Do not give mugs, ornaments, or decorative items that will just create clutter. One sweet teacher asked that we only give small, inexpensive gifts she could use in the classroom--stickers, sidewalk chalk, pencils, etc.
This all ends after elementary school. I don't think middle or high school teachers ever get much, if anything.