My ex keeps bad-mouthing me to our daughter

My 13yo came home last weekend and mentioned her mom told her I "don't care about spending money on her" because I won't pay for the $300 soccer camp. Like... I literally just paid for her club team uniform and cleats. This is frustrating because she's old enough to see through it but also young enough to internalize it. How do I handle this without seeming like I'm bad-mouthing back? I've been taking the high road but it's exhausting.