I would take whatever you can afford, and divide it by 4.
So, if you can afford $10, get 4 nice cards and write a nice note. if you can afford $20, get 4 picture frames, or buy the ingredients and make 4 batches of cookies, etc . . .
You don't need to spend more on gifts than a family whose kids don't have SN, but please don't give the TA's less.