If you can easily afford $50, I don't see a problem with that amount.
I would like to make another suggestion... perhaps not for this particular time, but you might want to consider it for a future gift. If you have a talent in making some item, or a special type of unique item that you can purchase, these often are treasured by teachers. Examples from my own experience: one little girl gave me a lovely bracelet that featured kids around it. I wore it constantly for a number of years until it finally wore out and broke.
Another family had a sideline business importing items from Mexico. Over the years that I worked with their daughter, I received a lovely serape' and a really nice piece of pottery for my patio.
Other children made handcrafted items, such as Christmas tree ornaments made from pine cones.
Whether the item was large and relatively expensive, or small, homecrafted and inexpensive, the love and thoughtfullness that was put into the gift meant a lot to me and I have treasured those gifts over the years.