The great thing about moms is that they appreciate ANYTHING. Flowers always work especially on a budget. Or get a card and stick a gift card in it and grab a teddy bear. The kind of stuff that you can get at Walmart, but doesn't necessarily scream "I got this last minute gift at Walmart last night."
Or you can make her a card with your handprint on it like from kindergarten. That'll make her cry.
Or if you have ingredients at your house, you can bake her cookies or a cake and write I love you, Mom on them.