Normally I spend $5-$10 and usually try to get something on sale so it has a higher value.
Several times I have bought a $5 video rental card and given it with microwave popcorn. Simple and it can be used up and you don't have to know what they like or don't like.
I had many gifts to buy with 4 kids in grade school so I had to go the cheap route. Unfortunately for the junior high and high school teachers, we haven't given many gifts because there are so many teachers. I would be open to suggestions in this area. Sometimes if they had a favorite teacher they brought something small.
I don't think that you need to spend much, but if you want to that's great, too.