Very generous gift, however, you have another child just behind so don't set that bar too high unless you are really dealing with an extraordinary teacher. You will still be purchasing an end of the year present too. I would say $15 - $20 is generous - going in with a friend and spending $40 would make a lovely present.
I've been a room mom many times. Something that I have always done is a "class gift". It really helps with expenses and still gives the teacher a very generous present. By requesting that people donate $5 - $10 (if people aren't giving another gift, they;ll give the $10; if they are already buying something small, many times they will give $5). For many parents and children, it's important to them to make sure that the teacher knows that they "bought" part of that gift (never listing how much their donation was!) so make sure they know up-front that they will sign a teacher's card or what I do is, in your request letter, include a label that they can sign and you will stick in the card.
Good luck.